South Pasadena • AP Chemistry
Name
1 ▪ Describing Matter
Period
1.3
PROBLEMS
–
Date
STOICHIOMETRY
1. A noncarbonated soft drink contains an unknown amount of citric acid, H3C6H5O7. If 100. mL of the soft
drink requires 33.51 mL of 0.0102 M NaOH to neutralize the citric acid completely, how many grams of citric
acid does the soft drink contain per 100. mL? The reaction of citric acid and NaOH is
H3C6H5O7 (aq) + 3 NaOH (aq) → Na3C6H5O7 (aq) + 3 H2O (ℓ)
(Ch 5, #86)
2. Vitamin C is the simple compound C6H8O6. Besides being an acid, it is also a reducing agent. One method
for determining the amount of vitamin C in a sample is there to titrate it with a solution of bromine, Br2, an
oxidizing agent.
C6H8O6 (aq) + Br2 (aq) → 2 HBr (aq) + C6H6O6 (aq)
Suppose a 1.00 g “chewable” vitamin C tablet requires 27.85 mL of 0.102 M Br2 for titration to the
equivalence point. How many grams of vitamin C are in the tablet? (Ch, 5, #90)
3. Sodium thiosulfate, Na2S2O3, is used as a “fixer” in black-and-white photography. Assume you have a bottle
of sodium thiosulfate and want to determine its purity. The thiosulfate ion can be oxidized with I2 according
to the equation
I2 (aq) + 2 S2O32– (aq) → 2 I– (aq) + S4O62– (aq)
If you use 40.21 mL of 0.246 M I2 in a titration, what is the weight percent of Na2S2O3 in a 3.232-g sample of
impure material? (Ch 5, #100)
4. Suppose you mix 25.0 mL of 0.234 M FeCl3 solution with 42.5 mL of 0.453 M NaOH. What is the maximum
mass, in grams, of Fe(OH)3 that precipitates? Which reactant is in excess? What is the molar concentration
of the excess reactant remaining in solution after the maximum mass of Fe(OH)3 has been precipitated?
(Ch. 5, #108)
5. You wish to determine the weight percent of copper in a copper-containing alloy. After dissolving a sample of
an alloy in acid, an excess of KI is added, and the Cu2+ and I– ions undergo the reaction
2 Cu2+ (aq) + 5 I– (aq) → 2 CuI (s) + I3– (aq)
The liberated I3– is titrated with sodium thiosulfate according to the equation
I3– (aq) + 2 S2O32– (aq) → S4O62– (aq) + 3 I–
If 26.32 mL of 0.101 M Na2S2O3 is required for titration to the equivalence point, what is the weight percent
of Cu in 0.251 g of the alloy? (Ch 5., #109)
6. Iron reacts with hydrochloric acid to produce iron(II) chloride and hydrogen gas:
Fe (s) + 2 HCl (aq) → FeCl2 (aq) + H2 (g)
The H2 gas from the reaction of 2.2 g of iron with excess acid is collected in a 10.0 L flask at 25°C. What is
the pressure of the H2 gas in this flask? (Ch. 12 #44)
7. Sodium azide, the explosive compound in car air bags, decomposes according to the equation
2 NaN3 (s) → 2 Na (s) + 3 N2 (g)
What mass of sodium azide is required to provide the nitrogen needed to inflate a 25.0 L bag to a pressure of
1.3 atm and 25°C? (Ch. 12, #46)
AP Chemistry 2008B #3
A 0.150 g sample of solid lead(II) nitrate is added to 125 mL of 0.100 M sodium iodide solution. Assume no
change in volume of the solution. The chemical reaction that takes place is represented by the following equation.
Pb(NO3)2(s) + 2 NaI(aq) → PbI2(s) + 2 NaNO3(aq)
(a) List an appropriate observation that provides evidence of a chemical reaction between the two compounds.
(b) Calculate the number of moles of each reactant.
(c) Identify the limiting reactant. Show calculations to support your identification.
(d) Calculate the molar concentration of NO3−(aq) in the mixture after the reaction is complete.
(e) Circle the diagram below that best represents the results after the mixture reacts as completely as possible.
Explain the reasoning used in making your choice.