MOLS, PERCENTS, and STOICHIOMETRY
Why do we need mols?
Percents by weight.
Basic stoichiometry.
Density times volume of a pure material.
Atoms or molecules to mols.
Concentration times volume of a
solution.
Gases.
Stoichiometry roadmap.
Mol and percent worksheet.
Stoichiometry problems with mass and gas at STP.
Stoichiometry problems on concentration and density.
Stoichiometry problems using complete roadmap.
WHY DO WE NEED MOLS?
Every chemist has dreamed that atoms were large enough to see
and manipulate one at a time. The same chemist realizes
after considering it, that if individual molecules were available
for manipulation, it would take far too long to get anything
done. The view from the atom is very different from the view of
trillions and trillions of atoms. The mass action of the atoms
that we see on our "macro" view of the world is the
result of the action of an incredibly large number of atoms
averaged in their actions. The most usual way we count the atoms is by
weighing them. The mass of material as weighed on a balance
and the atomic weight of the material being weighed is the way we
have of knowing how many atoms or molecules we are
working with. Instead of counting eggs, we can count cartons of
eggs, each carton of which has a given number, a dozen.
Instead of counting B-B's,
we can count liters of B-B's and find out how many B-B's are in a liter. Instead of
counting rice grains, we buy kilograms or pounds of rice and have an idea of how many
rice grains are in the container.
There are less than one hundred naturally occurring elements.
Each element has a characteristic atomic weight. Most
Periodic Charts include the atomic weight of an element in the
box with the element. The atomic weight is usually not an
integer because it is close to being the number of protons plus
the average number of neutrons of an element. Let's use the
atomic weight as a number of grams. This will give us the same
number of any atom we choose. If we weigh out 1.008
grams of hydrogen and 35.45 grams of chlorine and 24.3 grams of
magnesium, we will have the same number of atoms of
each one of these elements. The neat trick with this system is
that we can weigh the atoms on a grand scale of number of
atoms and get a count of them. This number of atoms that is the
atomic weight expressed in grams is Avogadro's number,
6.022 E 23. The
name for Avogadro's number of ANYTHING is a
mole or mol. A mol of aluminum is 27.0 grams of
aluminum atoms. Aluminum is a metal element, so the particles of
aluminum are atoms. There are Avogadro's number of
aluminum atoms in 27.0 grams of it. But 1.008 grams of hydrogen
is NOT a mol of hydrogen! Why not? Remember that
hydrogen is one of the diatomic gases. There is really no such
thing as loose hydrogen atoms. The total mass of a single hydrogen
diatomic molecule (H2) is 2. 016 AMU. A mol
of hydrogen gas has a mass of 2.016 grams. In that 2.016 gram
mass is Avogadro's number of H2 molecules
because that is the way hydrogen comes. A mol of water is 18.016 grams because
each water molecule has two hydrogen atoms and one oxygen atom. A
mol of water has in it Avogadro's number of water
molecules. Another way to view the same thing is that a formula weight
is the total mass of a formula in AMU expressed with units of grams per mol.
So Avogadro's number is just a number, like dozen or score or
gross or million or billion, but it is a very large number. You
could consider a mol of sand grains or a mol of stars. We are more
likely to speak of a mol of some chemical, for which we can
find the mass of a mol of the material by adding the atomic
weights of all the atoms in a formula of the chemical. The unit
of atomic weight or formula weight is grams/mol.
The chemical formula of a material should tell you; (a) which
elements are in the material, (b) how many atoms of each
element are in the formula, (c) the total formula weight, and (d)
how the elements are attached to each other. The symbols
of the elements tell you which elements are in the material. The
numbers to the right of each symbol tells how many atoms
of that element are in the formula. The type of atoms and their
arrangement in the formula will tell how the elements are
attached to each other. A metal and a nonmetal or negative
polyatomic ion shows an ionic compound. A pair of non-metals
are bonded by covalent bonds. Some crystals have water of
hydration loosely attached in the crystal. This is indicated by
the dot such as in blue vitriol, Cu(SO4)
· 5H2O, showing five molecules
of water of hydration to one formula of cupric sulfate.
The unit of the formula weight or molecular weight or
atomic weight is "grams per mol," so it provides a relationship
between mass in grams and mols of material.
nFw = m
'n' is the number of mols, 'Fw is the formula weight, and 'm' is the mass.
Back to the beginning of Mols, Percents, and Stoichiometry.
PERCENTS BY WEIGHT
All men weigh 200 pounds. All women weigh 125 pounds. What is
the percent by weight of woman in married couples? A married couple is one man and one
woman. (No political implications intended.) The total weight is 325
pounds. The formula for percent is:
In this case the woman is the target, so the weight of the woman
goes on top, and the total weight goes on the bottom of the fraction.
125 #
325 # |
x | 100% | = | 38.461% | = |
38.5% |
Notice that the units of pound cancel to make the percent a
pure number of comparison.
The weights of atoms are the atomic weights. What is the
percentage of chloride in potassium chloride? The atomic weight
of potassium is 39.10 g/mol. The atomic weight of chlorine is
35.45 g/mol. So the formula weight of potassium chloride is
74.55 g/mol. The chloride is the target and the potassium
chloride is the total. 35.45 g/mol / x 100% = 47.55198 % or
47.6 % to three significant figures.
35.45g/mol
74.55g/mol | x | 100% | = | 47.5520% | = | 47.6% |
You can do that with any part of a compound. What is the
percentage of sulfate in beryllium sulfate tetrahydrate?
Notice that the examples here are done to two decimal points
of the atomic weights. The problems in the practice bunch at
the end of this chapter are done to one decimal point of the
atomic weight.
Back to the beginning of Mols, Percents, and
Stoichiometry.
BASIC STOICHIOMETRY
Pronounce stoichiometry as "stoy-kee-ah-met-tree," if you want to sound like you know what you are talking about, or "stoyk," if you want to sound like a real geek.
Stoichiometry is just a five dollar idea dressed up in a fifty
dollar name. You can compare the amounts of any materials in the
same chemical equation using the formula weights and the
coefficients of the materials in the equation. Let's
consider the equation for the Haber reaction, the combination of
nitrogen gas and hydrogen gas to make ammonia.
N2 + 3 H2 —> 2 NH3
The formula
for nitrogen is N2 and the formula for
hydrogen is H2.
They are both diatomic gases. The formula for ammonia is NH3. The balanced equation requires one nitrogen molecule and
three hydrogen molecules to make two ammonia molecules, meaning that one
nitrogen molecule reacts with three hydrogen molecules to make two ammonia
molecules or one MOL of nitrogen and three MOLS of hydrogen make two MOLS
of ammonia. Now we are getting somewhere. The real way we measure amounts
is by weight (actually, mass), so 28 grams (14 g/mol times two atoms of
nitrogen per molecule) of nitrogen and 6 grams of hydrogen (1 g/mol times
two atoms of hydrogen per molecule times three mols) make 34 grams of ammonia.
Notice that no mass is lost or gained, since the formula weight for ammonia is
17 (one nitrogen at 14 and three hydrogens at one g/mol) and there are two mols
of ammonia made. Once you have the mass proportions, any
mass-mass stoichiometry can be done by good old proportionation.
What is the likelihood you will get just a simple mass-mass
stoich problem on your test? You should live so long. Well, you
should get ONE.
Rather than thinking in terms of proportions, think in mols
and mol ratios, a much more general and therefore more useful
type of thinking. A mol ratio is just the ratio of one material
in a chemical equation to another material in the same equation.
The mol ratio uses the coefficients of the materials as they
appear in the balanced chemical equation. What is the mol ratio
of hydrogen to ammonia in the Haber equation? 3 mols of hydrogen
to 2 mols of ammonia. Easy. In the standard stoichiometry
calculations you should know, ALL ROADS LEAD TO MOLS. You can
change any amount of any measurement of any material in the same
equation with any other material in any measurement in the same
equation. That is powerful. The setup is similar to Dimensional
Analysis, and the calculations can include portions of DA.
1. Start with what you know (GIVEN), expressing it as a fraction.
2. Use definitions or other information to change what you know to mols of that material.
3. Use the mol ratio to exchange mols of the material given to the mols of material you want to find.
4. Change the mols of material you are finding to whatever other measurement you need.
How many grams of ammonia can you make with 25 grams of
hydrogen? (Practice your mol math rather than doing this by
proportion. Check it by proportion in problems that permit it.)
You are given the mass of 25 grams of hydrogen. Start there.
25 g H2/1 Change to mols of hydrogen by
the formula weight of hydrogen 1 mol of H2 =
2.0 g. (The 2.0 g goes in the denominator to cancel with the gram units in the material given.)
Change mols of hydrogen to mols of ammonia by the mol
ratio. 3 mols of hydrogen = 2 mols of ammonia. (The mols of
hydrogen go in the denominator to cancel with the mols of
hydrogen. You are now in units of mols of ammonia.) Convert the
mols of ammonia to grams of ammonia by the formula weight of
ammonia, 1 mol of ammonia = 17 g. (Now the mols go in the
denominator to cancel with the mols of ammonia.) Cancel the
units as you go.
The math on the calculator should be the last thing you do.
2 5 ÷ 2 . 0
x 2 ÷ 3 x 1
7 = and the number you get (141.66667)
will be a number of grams of ammonia as the units in your
calculations show. Round it to the number of significant digits
your instructor requires (often three sig. figs.) and put into
scientific notation if required. Most professors suggest that
scientific notation be used if the answer is over one thousand or
less than a thousandth. The answer is 142 grams of
ammonia.
The calculator technique in the preceding paragraph
illustrates a straightforward way to do the math. If you include
all the numbers in order as they appear, you will have
less chance of making an error. Many times students have been observed
gathering all the numbers in the numerator, gathering all the
numbers in the denominator, presenting a new fraction of the
collected numbers, and then doing the division to find an answer.
While this method is not wrong, the extra handling of the numbers
has seen to produce many more errors.
See the Stoichiometry Roadmap for a way to
consider this idea graphically. This example starts at "mass
given" and goes through the mol ratio to "mass find."
Notice by the chart above we may get the number of mols of
material given if we change the mass by the formula weight,
but in our continuous running math problem, we don't have to
stop and calculate a number of mols. Students who insist on doing problems piecemeal tend to get more calculator errors.
The more traditional formula for converting mols to mass would
be, where Fw is the formula weight, m is the mass, and n is
the number of mols: n x Fw = m. You should be able to "see" these
formula relationships on the roadmap.
Back to the beginning of Mols, Percents, and
Stoichiometry.
DENSITY TIMES MASS OF A PURE MATERIAL
Density multiplied by the
volume of a pure material is equal to the mass of that material.
If we know the density of a material and the volume of the pure
material, with D = density and V = volume, DV = m so:
If you were given the density and volume of pure material you
could calculate the volume of another material in that
equation if you know its density. Notice that the density
must be inverted to cancel the units properly if you want the
volume to find. If you need to find the density, the volume must
be inverted.
See the Stoichiometry Roadmap for a
graphic
view of this idea. Start with "Given density times volume of a
pure material.
Back to the beginning of Mols, Percents, and
Stoichiometry.
ATOMS OR MOLECULES TO MOLS
One of the hardest ideas for some students is that the
individual particles of a material are a single one of a formula
of that material. Copper element comes only in the form of atoms.
Water only comes in the form of a molecule with one oxygen and
two hydrogen atoms. A mol, then is Avogadro's number of
individual particles of whatever type of pure material the
substance is made. There is no such thing as a mol of mud because
mud is a mixture. There is no one mud molecule.
The word "pure" also can be misunderstood. We do not mean that
a material is one hundred percent the same material for us to use
it, but that we are only considering the amount of that material.
The formula behind this relationship is: where n is the number
of mols, A is Avogadro's number, and # is the number of
individual particles of material,
A x n = #.
Refer to the Stoichiometry Roadmap for a
graphic view of this idea.
Back to the beginning of Mols, Percents, and
Stoichiometry.
CONCENTRATION TIMES VOLUME OF A
SOLUTION
A solution is a mixture of a fluid (often water, but
not
always) and another material mixed in with it. The material mixed
in with it is called the solute. There is more on
solutions in the chapter devoted to that. The volume of a
solution, V, is measured the same way the volume of a pure liquid
is measured. The concentration can be expressed in a number of
ways, the most common in chemistry is the M, molar. One molar is
one mol of solute in a liter of fluid. It is important to notice
that the fluid is usually nothing more than a diluting agent. For
most of the reactions, the fluid does not participate in any
reaction.
Concentration times volume is number of mols of the solute
material.
C x V = n
The "given" side of concentration times volume is easy. As
with density times volume of a pure material, but the "find" side
may need more work. You need one or the other of the
concentration and volume before you can calculate the other. At
the end of the Dimensional Analysis if you want concentration,
you will be using the volume inverted. If you want the volume,
you will be using the concentration inverted. This is not so
difficult because the units will guide you.
Refer to the Stoichiometry Roadmap for a
graphic view of this idea.
Back to the beginning of Mols, Percents, and
Stoichiometry.
GASES
Standard temperature is zero degrees Celsius. Standard
pressure is one atmosphere. A mol of ANY gas at standard
temperature and pressure (STP) occupies 22.4 liters. That number
is good to three significant digits. The equation would be 1 mol
gas = 22.4 L @STP. The conversion factor, the Molar Volume of
Gas, is 1 mol gas/22.4 L @STP or 22.4 L @STP/1 mol gas
Where n is the number of mols, V is the volume of a gas, and
MVG is the molar volume of gas,
V = n x MVG
Gases not at STP will require the Ideal Gas Law Formula,
P V = n R T
where P is the pressure of the gas in atmospheres, V is the
volume of the gas in liters, n is the number of mols of gas, T is
the Kelvin temperature of the gas, and R is the "universal gas
constant" with the measurement of 0.0821 liter-atmospheres per
mol-degree. We will have to do some algebra on the PV = nRT gas
equation to do the gas portion of the stoichiometry problems.
In GIVEN we only need to solve for n. n = PV/RT. If we need to
find the volume, pressure, or temperature of a gas, we need to
solve for the unknown and include the "mols find" as the n. More
about gases later. See the Chemtutor section on Gases
for math problems using the gas laws.
The earmarks of a stoichiometry problem are: There is a
reaction. (A new material is made.) You know the amount of one
material and you are asked to calculate the amount of another
material in the same equation.
Back to the beginning of Mols, Percents, and
Stoichiometry.
HOW TO USE THE "ROADMAP" FOR SOLVING CHEMISTRY
PROBLEMS
* 1. Write all the compounds and elements in the problem
correctly.
* 2. Write the balanced chemical equation for the problem.
* 3. Write the MATERIAL you have enough information about to
use as GIVEN. (This has been one of the major stumbling blocks in
using the roadmap.) If you know the number of moles, the mass, or
the number of molecules of a material, you have all you need to
start the problem. You need CONCENTRATION AND VOLUME of a
solution to have the amount of solute that reacts. You need
VOLUME AND DENSITY of a solid or liquid to have an amount of
that. You need VOLUME, PRESSURE AND TEMPERATURE of a gas to have
a complete set of information. (Notice it is useful to understand
the properties of the states of matter as you do this.)
* 4. Write what you need to FIND and all the other pertinent
information about that material. For instance, if you need to
find the volume of a gas, you must also list the pressure and
temperature of that gas in FIND. In this manner: FIND V, volume
of gas at 79°C and 1.8 atm.
* 5. Sketch out an outline of the math according to the
roadmap. You know there are some points in the roadmap that you
miss on the outline because they are calculated in the process,
for instance if you are given a mass of one material and asked to
find the density of another material with its volume, you would
start at the MASS GIVEN and use the FORMULA WEIGHT to get to the
MOLES GIVEN, but MOLES GIVEN does not appear in the outline
because it is already calculated. You next need the MOLE RATIO to
get the MOLES FIND. Again, MOLES FIND does not appear in the
outline.
* 6. Fill in the outline with the numbers, units and materials
(for instance, 15 kg Mg) and do the calculations. Be careful of
numbers that need to be inverted. You can tell the coefficients
that need to be inverted by the units.
STOICHIOMETRY ROADMAP
One of the really nice things about the Stoichiometry Roadmap
is that once you understand it thoroughly, it can be carried
around with you between your ears. Just remember that ALL ROADS
LEAD TO MOLS.
Back to the beginning of Mols, Percents, and
Stoichiometry.
MOLE AND PERCENT WORKSHEET
1. How many pennies are in a mole of pennies? How many
thousand-dollar bills (k-notes!) is that mole of pennies equal
to?
2. NO2 is the molecular formula for
nitrous dioxide (also known as nitrogen dioxide). List the
information available to you from this formula?.
3. C2H2 is the
molecular formula for ethyne (A.K.A. acetylene).
(a) How many atoms are in one molecule?
(b) Which atoms make up acetylene? (c) How many moles of atoms are in one
molecule of acetylene? (d) How many molecules are
in 5.3 moles of acetylene? (e) How many atoms are
in a mole of acetylene?
4. Calculate the molar mass of a mole of the following
materials: (a) Al (b) Ra (c) Co (d) CO (e) CO2 (f) HCl (g) Na2CO3 (h) Ca(NO3)2 (i) (NH4)3(PO4)
(j) H2O (k) Epsom salts -
Mg(SO4) · 7H2O (m)
blue vitriol - Cu(SO4) · 5H2O
?
5. Calculate the number of moles in: (a) 2.3 # of carbon (b)
0.014 g of Tin (c) a 5 Oz silver bracelet (d) a pound of table
salt (e) a 350 kg cast iron engine block (f) a gal. of water (8.3
#) (g) a ton of sand (SiO2) (h) 6.2 grams of
blue vitriol (i) a pound of Epsom salts ?
6. Calculate the number of atoms in: (a) 100 g of Argon (b)
1.21 kg aluminum foil (c) a 28 # lead brick (d) the E7 kg of water in
an Olympic swimming pool (e) 7 kg of hydrogen gas (f) a tonne of
calcium nitrate ?
7. What is the percentage composition of oxygen in each of the
following materials: (a) CO (b) CO2 (c)
(NO3)-
(d) isopropyl alcohol C3H8O
(e) calcium nitrate (f) blue vitriol - Cu(SO4) · 5H2O ?
8. What is the percentage composition of phosphate in each of
the following materials: (a) phosphoric acid (b) sodium
carbonate (c) ammonium phosphate (d) calcium phosphate ?
9. What is the percentage composition of sulfate in each of
the following materials: (a) sulfuric acid (b) sodium sulfate (c)
Epsom salts ( d) aluminum sulfate ?
ANSWERS TO MOL AND PERCENT PROBLEMS
| 1a. 6.023 E23 pennies | 1b. 6.023 E18 k-Notes | 2a. Covalent |
| | | | |
| 2b. Elements in it (N
and O) | 2c. Number of
atoms of each
element |
| | | | |
| 3a. 4 | 3b. C &
H | 3c. 6.64 E-24 | 3d.
3.1922 E24 | 3e. 2.4092 E24 |
| | | | |
| 4a. 27.0 | 4b.
226.0 | 4c. 58.9 | 4d.
28.0 | 4e. 44.0 |
| | | | |
| 4f. 36.5 | 4g.
106.0 | 4h. 164.1 | 4i.
149.0 | 4j. 18.0 |
| | | | |
| 4k. 246.4 | 4m.
249.6 | 5a. 86.9 | 5b. 1.18
E-4 | 5c. 1.31 |
| | | | |
| 5d. 7.75 | 5e. 6.27 E3 | 5f. 210
| 5g. 1.51 E4 | 5h. 0.0248 |
| | | | |
| 5i. 1.84 | 6a. 1.51 E24 | 6b. 2.69
E25 | 6c. 3.69 E25 | 6d. 1.00 E33 |
| | | | |
| 6e. 4.22E27 | 6f. 3.30 E28 | 7a.
57.1% | 7b. 72.7% | 7c. 77.4% |
| | | | |
| 7d. 26.7% | 7e. 58.5% | 7f. 57.7%
| 8a. 96.9% | 8b. 0% |
| | | | |
| 8c. 63.8% | 8d. 61.2% | 9a. 98.0%
| 9b. 67.6% | 9c. 39.0% |
| | | | |
| 9d. 84.2% | |
| | |
| | | | |
Back to the beginning of Mols, Percents, and
Stoichiometry.
STP GAS AND MASS STOICHIOMETRY PROBLEMS (PRELIMINARY TO GAS
LAW)
All of the problems below are stoichiometry problems with at
least one equation participant as a gas at STP. (a) Write and
balance the chemical equation. (2) Do the math in DA style using
1 mole gas at STP = 22.4 liters as a factor. In the following
problems ALL GASES ARE AT STP. Click here for a general idea of how to do the problems
in this set.
1. How many moles of nitrogen gas is needed to react with 44.8
liters of hydrogen gas to produce ammonia gas?
2. How many liters of ammonia are produced when 89.6 liters of
hydrogen are used in the above reaction?
3. Ten grams of calcium carbonate was produced when carbon
dioxide was added to lime water (calcium hydroxide in solution).
What volume of carbon dioxide at STP was needed?
4. When 11.2 liters of hydrogen gas is made by adding zinc to
sulfuric acid, what mass of zinc is needed?
5. What volume of ammonia at STP is needed to add to water to
produce 11 moles of ammonia water?
6. How many grams of carbonic acid is produced when 55 liters
of carbon dioxide is pressed into water?
7. magnesium hydroxide + ammonium sulfate 
magnesium sulfate + water +
ammonia
How much (grams) magnesium hydroxide do you need to use in the
above reaction to produce 500 liters of ammonia?
8. How much strontium bromide is needed to add to chlorine gas
to produce 75 liters of bromine?
9. What mass of ammonium chlorate is needed to decompose to
give off 200 liters of oxygen?
10. Your car burns mostly octane, C8H18, as a fuel. How many
liters of oxygen is needed to burn a kilogram of octane?
11. copper + sulfuric acid copper II sulfate
+ water + sulfur dioxide
How many moles of copper are needed to produce 1000 L of
SO2?
12. What volume of oxygen is needed to burn a pound of
magnesium?
13. How many grams of sodium do you have to put into water to
make 30 liters of hydrogen at STP?
14. ammonia gas and hydrogen chloride gas combine to make
ammonium chloride. What volume of ammonia at STP is needed to
react with 47.7 liters of hydrogen chloride at STP?
15. How many liters of oxygen are needed to burn 10 liters of
acetylene?
ANSWERS TO STP GAS AND MASS STOICHIOMETRY PROBLEMS
| 1. 0.667 mol | 2. 59.7
L | 3. 2.24 L | 4. 32.7 g
|
| | | |
| 5. 246 L | 6. 152 g | 7. 651 g | 8. 828 g |
| | | |
| 9. 604 g | 10. 2.46 kL | 11. 44.6 mol | 12. 210 L |
| | | |
| 13. 61.6 g | 14. 47.7 L | 15. 25
L | |
Back to the beginning of Mols, Percents, and
Stoichiometry.
PROBLEMS ON CONCENTRATION AND DENSITY
WRITE AND BALANCE THE CHEMICAL EQUATION FOR THOSE PROBLEMS
THAT NEED IT. SHOW ALL YOUR WORK. USE W5P OR DA METHOD ACCORDING
TO THE ROADMAP.
1. The lead brick on my desk measures 3 by 5 by 11 cm. Lead
has a density of 11.34 g/cc. How many lead atoms are in that
block?
2. The lab technician at the peanut packing factory
takes a bag of peanuts, puts water into it to dissolve the salt,
and dilutes the solution to one liter. She then takes ten ml of
that solution and titrates it against 0.132 M silver nitrate. One
bag sample takes 31.5 ml of silver nitrate to endpoint. What mass
of salt was in the bag?
3. What is the concentration of sugar (C12H22O11)
if twenty grams are dissolved in enough water to make 2
liters?
4. Methyl alcohol (CH3OH) has a density of
0.793 kg/l. What volume of it is needed to add to water to make
five liters of 0.25 M solution?
5. Magnesium has a density of 1.741 g/cc. What volume of Mg
will burn in 20 liters of oxygen at 2.1 atm and 0°C?
6. Uranium metal can be purified from uranium hexafluoride by
adding calcium metal. Calcium metal has a density of 1.54 g/cc.
Uranium has a density of 18.7 g/cc. What mass of uranium do you
get for a Kg of Ca? What volume of uranium do you get for a cubic
meter of calcium?
7. What volume of 0.27 M sodium hydroxide is needed to react
with 29.5 ml of 0.55 M phosphoric acid?
8. What volume of carbon dioxide is produced at 1 atm and
87 °C when 1.6 liters of methyl alcohol burns? What volume of
liquid water is produced in this reaction?
9. Seven kilograms of mercury II oxide decomposes into mercury
and oxygen. Mercury has a density of 13.6 g/cc/ What volume of
mercury is produced?
10. Water and calcium oxide produce calcium hydroxide. How
many grams of calcium hydroxide are made if you add 275 liters of
water to enough calcium oxide?
11. Gasoline (C7H16)
has a density of 0.685 kg/liter. How many liters of oxygen at
37 °C and 950 mmHg are needed to burn 15 liters of gasoline?
12. Sodium hydroxide and hydrochloric acid combine to make
table salt and water. 14 mL of 0.1 M sodium hydroxide is
added to an excess of acid. How many moles of table salt are
made? How many grams of salt is that?
13. 50 mL of 0.25 M copper II sulfate evaporates to leave
CuSO4 · H2O. (That is the pentahydrate crystal of copper II
sulfate.) What is the mass of this beautiful blue crystal from
the solution?
14. Chlorine gas is bubbled into 100 mL of 0.25 M potassium
bromide solution. This produces potassium chloride and
bromine gas. The bromine (which dissolves in water) is taken from
the solution and measured at 27 °C and 825 mmHg.
What is the volume of bromine?
15. 95.0 mL of 0.55 M sulfuric acid is put on an excess of
zinc. This produces zinc sulfate and hydrogen. How many grams of
zinc sulfate are made?
16. 27.6 mL of a 0.190 M solution of silver nitrate and 15.4
mL
of an unknown (but excess) amount of sodium chloride
combine to make a white precipitate silver chloride and some
dissolved sodium nitrate. (a) How many moles of silver
chloride are made? (b) How many grams of silver chloride is that?
(c) How many moles of sodium nitrate are made? (d)
What is the concentration of sodium nitrate in the final
solution?
17. How many grams of potassium permanganate, KMnO4, is
needed to make 1.72 liters of 0.29 M solution?
18. By my calculations, a drop of ethyl alcohol, C2H5OH , in
an Olympic-sized swimming pool produces a 1.20 E-10 M
solution of alcohol in water. A drop is a twentieth of a mL. How
many molecules of ethyl alcohol are in a drop of the water
in the pool?
19. 93.0 mL of 0.150 M magnesium hydroxide is added to 57.0 mL
of 0.4 M nitric acid. (Magnesium nitrate and water are
formed. What is the concentration of the magnesium nitrate after
the reaction?
ANSWERS TO PROBLEMS ON CONCENTRATION AND
DENSITY
| 1. 5.44 E24 atoms | 2. 24.3 g
| 3. 0.0292 M | 4. 0.0504 L
|
| | | |
| 5. 52.3 ml(cc) Mg | 6a.
1.98 kg of U | 6b. 1.63 E6 mL | 7. 180 mL |
| | | |
| 8a. 1.17 kL CO2 * | 8b. 1.43 L | 9. 0.477 L | 10. 1.13 E 6 g |
| | | |
| 11. 23.0 kL * | 12a. 1.4 E-3 mols
| 12b. 0.0819 g | 13. 3.12 g |
| | | |
| 14. 284 mL | 15. 8.44 g
| 16a. 5.24E-3 mol | 16b.
0.752 g |
| | | |
| 16c. 5.24E-3 mols | 16d. 122
mmolar | 17. 78.8 mg | 18.
3.61E9
molecules |
| | | |
| 19. 0.152 M | | |
* The unit "kL" or "kiloliter" is not a recognized SI unit. It does calculate to the same volume as a cubic meter, m3.
Back to the beginning of Mols, Percents, and
Stoichiometry.
PROBLEMS USING COMPLETE ROADMAP
1. How many liters of ammonia at 0°C and 25
atm. are produced when 10 g of hydrogen is combined with
nitrogen?
2. How many milliliters of hydrogen at 0°C and 1400 mmHg
are made if magnesium reacts with 15 mL of 6 M sulfuric
acid?
3. How many atoms are in 25 liters of fluorine gas at 2.85 atm
and 450°C?
4. Liquid butane (C4H10)
has a density of 0.60 g/cc. It burns to
make carbon dioxide at 120°C. What volume of carbon
dioxide is produced at one atm when 350 liters of liquid butane
burns?
5. Isopropyl alcohol, C3H7OH, makes a good fuel for cars. What
volume of oxygen at 785 mmHg and 23°C is needed to
burn 8.54 E25 molecules of isopropyl alcohol?
6. How many moles of NaCl are in a liter of a 0.15 M NaCl
solution? (0.15 M NaCl is physiological saline when
sterilized.)
7. How many grams of NaCl must you put into a 50 liter
container to make a physiological saline solution?
8. Chlorine gas is bubbled into 100 mL of 0.25 M potassium
bromide solution. This produces potassium chloride and
bromine gas. The bromine dissolves completely in the water. What
is the concentration of bromine?
9. 95 mL of 0.55 M sulfuric acid is put on an excess of zinc.
This produces zinc sulfate and hydrogen. How many grams of
zinc sulfate are made?
10. Methyl alcohol (CH3OH) has a density
of 0.793 Kg/L. What volume of it is needed to add to water to
make twenty-five liters of 0.15 M solution?
11. Magnesium has a density of 1.741 g/cc. What volume of Mg
will burn to produce a kilogram of magnesium oxide?
12. What volume of water vapor is produced at 716 mmHg and
87°C
when 2.6 liters of methyl alcohol burns?
ANSWERS TO PROBLEMS USING COMPLETE
ROADMAP
| 1. 2.99 L | 2. 1.10 E3
mL | 3. 1.45 E24 atoms | 4. 4.67
E5 L |
| | | |
| 5. 1.50 E4 L | 6. 0.15 moles
| 7. 439 g | 8. 0.125
M |
| | | |
| 9. 8.44 g | 10. 151 mL
| 11. 0.346 L | 12. 1.29 E5 L
|
Heuristics
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Units and Measures
Atomic Structure
Elements
Periodic Table
States of Matter
Compounds
Reactions
Oxidation and Reduction Reactions
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Thermochemistry
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