A Little Review . . . ANSWER KEY
Directions: Read each of the following questions carefully and answer in the space provided. If a question
requires a calculation, SHOW ALL WORK! No work, no credit, no kidding! Remember to use units and significant
figures where appropriate. Good luck!
Chemical Reactions
1. Fill in the blanks below with the name of one of the five types of chemical reactions: synthesis, decomposition,
single displacement, double displacement, and combustion.
a) one reactant breaks down into two or more products
decomposition
b) element + compound → element + compound
single displacement
c) hydrocarbon + oxygen → carbon dioxide + water
combustion
d) two ions trade places with each other in compounds
double displacement
e) two or more substances make one product
synthesis
2. Identify the TYPE of chemical reaction below.
a) 2 H2 + O2 → 2 H2O
synthesis (one PRODUCT)
b) 2 KIO3 → 2 KI + 3 O2
decomposition (one REACTANT)
c) Fe + CuSO4 → FeSO4 + Cu
single displacement
d) Li2SO4 + Pb(NO3)2 → 2 LiNO3 + PbSO4
double displacement
e) C3H8 + 5 O2 → 3 CO2 + 4 H2O
combustion
The Mole and Mole Conversions
3. The SI unit used to measure an amount is a(n) mole.
4. What is molar mass? The mass (in grams) of one mole of a substance. Unit is g/mol.
5. Calculate the molar mass of:
a) CuSO4
Cu = 1 x 63.5 = 63.5
S = 1 x 32.1 = 32.1
O = 4 x 16 = 64
159.6 g/mol CuSO4
b) octane
CnH2n + 2
C8H2(8) + 2 = C8H18
C = 8 x 12 = 96
H = 18 x 1 = 18
114 g/mol C8H18
6. a) molar mass (g) = 1 mole = 6.02 x 1023 particles (atoms, molecules, formula units)
b) 6.02 x 1023 is called Avogadro’s number
n = # C (oct = 8)
7. How many moles are in 315 grams of magnesium chloride?
Need: mol MgCl2
Given: 315 g MgCl2
315 g MgCl2 x 1 mol MgCl2 = 315 = 3.3053515 = 3.31 mol MgCl2
1
95.3 g MgCl2
95.3
1 mol = molar mass
8. How many grams are in 7.6 moles of dinitrogen trioxide?
Need: g N2O3
Given: 7.6 mol N2O3
7.6 mol N2O3 x 76 g N2O3 = 577.6 = 580 g N2O3
1
1 mol N2O3
1 mol = molar mass
N = 2 x 14 = 28
O = 3 x 16 = 48
76 g/mol
9. How many atoms are in 8.40 moles of aluminum (Al)?
Need: atoms Al
Given: 8.40 mol Al
8.40 mol Al x 6.02 x 1023 atoms Al = 5.0568 x 1024 = 5.06 x 1024 atoms Al
1
1 mol Al
1 mol = 6.02 x 1023 particles
10. How many molecules are in 88.0 grams of carbon dioxide?
Need: molecules CO2
Given: 88.0 g CO2
88.0 g CO2 x 1 mol CO2 x 6.02 x 1023 molecules CO2 = 5.2976 x 1025 = 1.204 x 1024 = 1.20 x 1024
1
44 g CO2
1 mol CO2
44
molecules CO2
1 mol = molar mass
C = 1 x 12 = 12
O = 2 x 16 = 32
44 g/mol
1 mol = 6.02 x 1023 particles
11. How many moles are in 7.05 x 1024 atoms of carbon (C)?
Need: mol C
Given: 7.05 x 1024 atoms C
7.05 x 1024 atoms C x
1 mol C
1
6.02 x 1023 atoms C
= 7.05 x 1024 = 11.71096346 = 11.7 mol C
6.02 x 1023
1 mol = 6.02 x 1023 particles
12. How many grams are in 6.02 x 1023 formula units of LiCl?
Need: g LiCl
Given: 6.02 x 1023 f.u. LiCl
6.02 x 1023 f.u. LiCl x
1
1 mol LiCl
x 42.4 g LiCl = 2.55248 x 1025 = 42.4 f.u. LiCl
6.02 x 1023 f.u. LiCl
1 mol LiCl
6.02 x 1023
1 mol = 6.02 x 1023 particles
1 mol = molar mass
13. a) What is an empirical formula? The simplest (most reduced) version of the formula.
b) What is a molecular formula? The actual (real) formula that is NOT reduced; the molecular formula
CAN be the same as the empirical formula. Water’s actual/real formula is H 2O – this is the
molecular formula. H2O cannot be reduced any further so it is also the empirical formula.
14. Compare the following formulas: C4O2H8 and C2OH4.
a) Which is the empirical formula? C2OH4 Why? It’s the simplified/reduced formula.
b) Which is the molecular formula? C4O2H8 Why? It’s the actual/real formula that is unreduced.
15. a) What is the empirical formula for C6H14? C3H7
b) What is the empirical formula for H2O2? HO
16. Explain how to find an empirical formula when given the percent composition of each element in the compound.
Use the following rhyme:
 Percent to mass
 Mass to mole
 Divide by smallest
 Multiply till whole
17. a) Determine the empirical formula for a compound containing 1.203% of calcium and 2.128% of chlorine.
1.203 g Ca x 1 mol Ca = 1.203 = 0.03 mol Ca = 1 Ca
1
40.1 g Ca
40.1
0.03 mol
2.128 g Cl x 1 mol Cl = 2.128 = 0.0599436 mol Cl = 1.99812 = 2 Cl
1
35.5 g Cl
35.5
0.03 mol
Empirical Formula becomes CaCl2
b) A compound has the empirical formula CH2O. If its molar mass is 150 g/mol, what is the molecular formula
of the compound?
n = molecular molar mass
empirical molar mass
n = 150 g/mol
30 g/mol
n = 5
n(empirical formula) = molecular formula
5(CH2O) = C5H10O5 = molecular formula
CH2O (empirical formula)
C = 1 x 12 = 12
H = 2 x 1 = 2
O = 1 x 16 = 16
30 g/mol (this is the empirical molar mass)
Stoichiometry
18. a) What is stoichiometry? Stoichiometry is the quantitative study (using numbers) of amounts of products
made and reactants used in chemical reactions.
b) Upon what is stoichiometry based? Law of Conservation of Mass/Matter states that matter/mass can
never be created nor destroyed only rearranged. This means that the mass of the reactants will
always equal the mass of the products. It also means that the number of atoms you start with on
the reactant side have to be the same number and kind of atoms that you end up with on the product
side.
19. a) What is a mole ratio? A mole ratio is a comparison of two numbers of moles. For example, you
compare the number of moles of one substance in a chemical equation to the number of moles of a
different substance in the same chemical equation.
b) Should a mole ratio ever be reduced? No. Explain why or why not. The number in the mole ratio comes
from the coefficient in the balanced chemical equation. The coefficient (the big number in front) IS
the number of moles. If you reduce the mole ratio, then you are changing the coefficient. The
coefficient was put there to balance the equation; therefore, if you change the coefficient (which is
shown in the mole ratio), you unbalance the equation!
20. Use the chemical reaction below to answer the following questions.
4 FeCl3 + 3 O2 → 2 Fe2O3 + 6 Cl2
a) What is the mole ratio between iron (III) chloride and chlorine in the equation below?
4 mol FeCl3
6 mol Cl2
**The substance mentioned first in the problem goes on TOP!
b) What is the mole ratio between chlorine and oxygen?
6 mol Cl2
3 mol O2
21. Balance the equation below, and then use it to answer the following questions.
2 LiOH + 1 PbSO4 → 1 Pb(OH)2 + 1 Li2SO4
a) How many moles of Pb(OH)2 will be produced from 3.0 moles of LiOH assuming you have an excess of PbSO 4.
Need: mol Pb(OH)2
Given: 3.0 mol LiOH
3.0 mol LiOH x 1 mol Pb(OH)2 = 3 = 1.5 mol Pb(OH)2
1
2 mol LiOH
2
mole ratio
# mol = coeff
b) If you need to make 750 g of Pb(OH)2, how many moles of LiOH would you need?
Need: mol LiOH
Given: 750 g Pb(OH)2
750 g Pb(OH)2 x 1 mol Pb(OH)2 x 2 mol LiOH =
1
241.2 g Pb(OH)2
1 mol Pb(OH)2
1 mol = molar mass
mole ratio
# mol = coeff
1500 = 6.2189054 = 6.2 mol LiOH
241.2
c) What mass (in grams) of Li2SO4 will be produced with 72.0 grams of LiOH?
Need: g Li2SO4
Given: 72.0 g LiOH
72.0 g LiOH x 1 mol LiOH x 1 mol Li2SO4 x 109.9 g Li2SO4 = 7912.8 = 165.53974 = 166 g Li2SO4
1
23.9 g LiOH
2 mol LiOH
1 mol Li2SO4
47.8
1 mol = molar mass
mole ratio
# mol = coeff
1 mol = molar mass
d) What mass (in grams) of PbSO4 is required to make 4.00 moles of Li2SO4?
Need: g PbSO4
Given: 4.00 mol Li2SO4
4.00 mol Li2SO4 x 1 mol PbSO4 x 303.3 g PbSO4 = 1213.2 = 1210 g PbSO4
1
1 mol Li2SO4
1 mol PbSO4
1
mole ratio
# mol = coeff
1 mol = molar mass
Limiting and Excess Reactant Stoichiometry
22. a) What is a limiting reactant? The reactant that runs out first in a chemical reaction; this reactant
determines the number of chemical reactions that take place and the amount of the product made
because once it runs out, no more chemical reactions occur and no more products can be made! The
LR also determines how much of the excess reactant is actually used up and left over because once
the LR runs out, no more of the other reactant can be used because the chemical reaction stops.
b) What is an excess reactant? This is the reactant that you have left over unused after the limiting
reactant runs out!
23. You are making one ham sandwich at home for lunch. The recipe requires that you have 3 slices of ham, 2
pieces of bread, 2 pieces of Swiss cheese, and 1 tablespoon of mayonnaise. You find that you have 9 slices of
ham, 4 slices of bread, 2 pieces of Swiss cheese, and 10 tablespoons of mayonnaise. Which ingredient would
be the limiting reactant?
9 slices ham x 1 sandwich = 9 = 3 sandwiches
1
3 slices ham
3
4 pieces of bread x 1 sandwich
= 4 = 2 sandwiches
1
2 pieces of bread
2
2 pieces Swiss cheese x 1 sandwich
= 2 = 1 sandwich
1
2 pieces Swiss cheese
2
10 tablespoons of Mayo x 1 sandwich
= 10 = 10 sandwiches
1
1 tablespoon of Mayo
1
Swiss Cheese is the LR because
you can make the fewest sandwiches!
24. Examine the general chemical reaction below, and use it to answer the question that follows.
3A + B → C
RECIPE (WHAT IS REQUIRED IN MOLES B/C COEFF = # MOLES)
If you have 9 moles of A and 5 moles of B, which would be the limiting reactant?
9 mol A x
1
What you
HAVE
1 reaction = 9 = 3 reactions with A – Limiting Reactant b/c fewest reactions
3 mol A
3
What is
Required Per
Reaction
5 mol B x 1 reaction = 5 = 5 reactions with B
1
1 mol B
1
What you
HAVE
What is
Required Per
Reaction
**Remember: # mol comes from the coefficient in the balanced chemical equation!
25. Examine the general chemical reaction below, and use it to answer the question that follows.
X2 + 2 Y → 2 XY
RECIPE (WHAT IS REQUIRED IN MOLES B/C COEFF = # MOLES)
If you have 7 moles of X2 and 8 moles of Y, which would be the limiting reactant?
7 mol X2 x 1 reaction = 7 = 7 reactions with X2
1
1 mol X2
1
What you
HAVE
What is
Required Per
Reaction
8 mol Y x 1 reaction = 8 = 4 reactions with Y – Limiting Reactant b/c fewest reactions
1
2 mol Y
2
What you
HAVE
What is
Required Per
Reaction
Solutions
26. What is a solution? A homogeneous mixture in which the parts are physically combined.
27. a) What is a solute? The substance being dissolved
b) What is a solvent? The substance doing the dissolving
28. You have a 25% aqueous solution of potassium bromide. Identify the solute and solvent.
Solute: potassium bromide
Solvent: water (aqueous means dissolved in water and water is the universal solvent)
29. a) What substance is nicknamed the “universal solvent”? water
b) Why is this substance given this nickname? Water (H2O) is polar because it has opposite charges (H+
and O-2) yet it is covalent because it is made of two or more nonmetals. Because water is polar
covalent, it has a unique ability to dissolve any polar solute as well as some nonpolar solutes (which
are covalent). This can be explained by the rule of thumb “like dissolves like.” This means that the
polarity of the solute and solvent must match. Therefore, since water is polar it can dissolve any
polar solute. It also explains why water can dissolve some nonpolar solutes because water is covalent
and nonpolar molecules are covalent. Because water can dissolve any polar solute and some nonpolar
solutes, it is called the “universal” solvent since it can dissolve so many things!
30. a) Explain the phrase “like dissolves like” using the words polar and nonpolar. “Like dissolves like” simply
means that the polarity of the solute and the solvent must match. A polar solvent can dissolve a polar
solute; likewise, a nonpolar solvent can dissolve a nonpolar solute.
b) Explain why oil and water do not mix to form a solution. Water is polar covalent. Oil is nonpolar.
This assumption can be made because we know “like dissolves like”. Polar solvents like water
can only dissolve other polar solutes. Since water does NOT dissolve the oil, we can then
surmise that the oil’s polarity is NOT like that of water. Since we know water is polar, the
oil must be nonpolar. Polar solvents like water cannot dissolve nonpolar solutes like the oil.
31. a)
b)
c)
d)
Ionic solutes dissolve best in which type of solvent: ionic or covalent? Ionic and polar
Covalent solutes dissolve best in which type of solvent: ionic or covalent? Covalent and nonpolar
Would sodium chloride dissolve best in water or hexane? water
Would carbon tetrachloride dissolve best in water or hexane? hexane
32. The three main types of solutions are saturated, unsaturated, and supersaturated. Use the description below
to identify the type of solution described.
a) To which type of solution could you add more solute to it and have it dissolve simply by agitating it?
unsaturated
b) Which type of solution will have some solute remaining unchanged at the bottom of the container?
saturated
c) Which type of solution dissolves more than the maximum amount of solute by increasing temperature?
supersaturated
33. Use the graph below to answer the following questions.
Solubility as a Function of Temperature
a) How much KCl dissolves in 100 g of water at 80C?
50 g KCl
b) 90 g of NH4Cl dissolves at 90C in 100 g of water. Is
this solution saturated, unsaturated, or supersaturated?
supersaturated
c) Which of the solutes on the graph behave as gases?
Ce2(SO4)3 and NH3
d) At 80C, a saturated solution of potassium chlorate is
made. If the solution cools to 30C, how much solute will
precipitate out?
@80C 40 g KClO3 dissolves then temp. drops to
30C & 10 g KClO3 dissolves
30 g KClO3 will NOT dissolve when temp
drops so it precipitates out
e) How much cerium sulfate will dissolve in 300. g of water at 20C?
10 g Ce2(SO4)3 =
x g Ce2(SO4)3
CROSS MULTIPLY TO SOLVE!
100 g H2O
300. g H2O
(100 g H2O)(x) = (10 g Ce2(SO4)3)(300. g H2O)
(100 g H2O)(x) = 3000 g Ce2(SO4)3  g H2O
100 g H2O
100 g H2O
x = 30 g Ce2(SO4)3
34. What is the molarity of a solution containing 6.0 moles of solute dissolved in 2.00 L of solution?
M = moles solute
L solution
M = ?
mol = 6.0 mol
L = 2.00 L
M = 6.0 mol
2.00 L
M = 3.0 mol/L = 3.0 M
35. What volume of solution is needed to make a 3.0 M solution containing 109.5 g of HCl?
M = moles solute
3.0 mol/L = 3 mol
Must convert grams to moles
L solution
x
because Molarity uses moles of
(3.0 mol/L)(x) = 3 mol
solute not grams of solute!
M = 3.0 mol/L
3.0 mol/L
3 mol/L
mol =
x = 1.0 L
L = ?
109.5 g HCl x 1 mol HCl = 109.5 = 3 mol HCl
1
36.5 g HCl
36.5
36. a) What is a colligative property? A physical property that depends on the number/amount of solute
particles not the identity of the solute.
b) Label whether the following increase or decrease when a solute is added to a pure solvent:
i. Freezing point
ii. Boiling point
iii. Vapor pressure
decreases (called freezing point depression)
increases (called boiling point elevation)
decreases
Acids and Bases
37. List three physical properties of an acid and one chemical property.
physical property: taste sour
physical property: turns blue litmus red and red litmus stays red; pH 0-6.9
physical property: turns pH paper red to orange
chemical property: reacts with bases AND reacts with metals AND reacts with carbonates
38. List three physical properties of a base and one chemical property.
physical property: taste bitter
physical property: turns red litmus blue and blue litmus stays blue; pH 7.1-14; pink in phenolphthalein
physical property: slippery feeling
chemical property: reacts with acids
39. a) Distinguish between a strong acid and a weak acid.
Strong acids dissociate ~100% and light up a conductivity
apparatus (light bulb) very brightly.
Weak acids dissociate only partially (far less than 100%)
and light up a conductivity apparatus (light bulb) dimly.
b) Distinguish between a strong base and a weak base.
STRONG
WEAK
Strong bases dissociate ~100% and light up a conductivity
apparatus (light bulb) very brightly.
Weak bases dissociate only partially (far less than 100%)
and light up a conductivity apparatus (light bulb) dimly.
40. Compare and Contrast an Arrhenius acid and base and a Bronsted-Lowry acid and base by filling in the chart
below.
Acid
Base
Arrhenius Definition
Contains hydrogen (H+) ions
Contains hydroxide (OH-) ions
Bronsted-Lowry Definition
Donates/gives away hydrogen ions
Accepts/takes hydrogen ions
41. Read the reaction below from left to right. Identify the acid, base, and conjugate acid (CA) and conjugate
base (CB). Remember: Conjugate just means the “opposite”. If you start with an Acid, the conjugate is the
base (and vice versa)
HCl
+
Acid
Donated H
NH3
↔
Base
+
Accepts H
NH4+
CA
+
ClCB
+
42. Examine the description below and determine whether it describes a strong acid, weak acid, strong base, or
weak base.
a) dissociates completely (~100%) and turns blue litmus red
strong acid
b) partially dissociates, stays clear in phenolphthalein, orange on pH paper
weak acid
c) dissociates completely (~100%) into OH ions
strong base
d) dimly lights up conductivity apparatus (light bulb) and turns pink in phenolphthalein
weak base
e) brightly lights up conductivity apparatus (light bulb) and has a pH of 2.1
strong acid
f) dissociates completely (~100%) into H+ ions
strong acid
g) dimly lights up conductivity apparatus (light bulb) and turns red litmus blue
weak base
h) dissociates completely (~100%), turns red litmus blue, and has a pH of 12.9
strong base
43. a) What type of double displacement reaction occurs when an acid reacts with a base? neutralization
b) What are ALWAYS the two products when an acid reacts with a base? salt and water
44. If you spilled oven cleaner containing sodium hydroxide on the kitchen counter, what could be used to
“counteract” this spill? Any acid because sodium hydroxide (NaOH) is a base. Acids and bases react
together and neutralize or “counteract” each other.
45. A flask contains 1 L of 5 M Mg(OH)2. Which of the following would completely “counteract” this solution?
a) 1 L of 5 M HCl
b) 1 L of 5 M Mg(OH)2
c) 0.5 L of 2.5 M HCl
d) 0.5 L of 2.5 M Mg(OH)2
46. Draw the pH scale from 0 to 14 and label the acidic, basic, and neutral pH’s.
Acids are from 0-6.9
7 = neutral
47. What is another word for a basic solution? alkaline
7.1-14 is basic (alkaline)