Ap Exam Prep Stoichiometry

AP Exam Prep: Stoich
1991 B
The molecular formula of a hydrocarbon is to be determined by analyzing its combustion products and
investigating its colligative properties.
(a) The hydrocarbon burns completely, producing 7.2 grams of water and 7.2 liters of CO
at standard
2
conditions. What is the empirical formula of the hydrocarbon?
(b) Calculate the mass in grams of O
required for the complete combustion of the sample of the hydrocarbon
2
described in (a).
(c) The hydrocarbon dissolves readily in CHCl
. The freezing point of a solution prepared by mixing 100.
3
grams of CHCl
and 0.600 gram of the hydrocarbon is -64.0C. The molal freezing-point depression
3
constant of CHCl
is 4.68C/molal and its normal freezing point is -63.5C. Calculate the molecular weight
3
of the hydrocarbon.
(d) What is the molecular formula of the hydrocarbon?
2001 B
Answer the following questions about acetylsalicylic acid, the active ingredient in aspirin.
(a) The amount of acetylsalicylic acid in a single aspirin tablet is 325 mg, yet the tablet has a mass of 2.00 g.
Calculate the mass percent of acetylsalicylic acid in the tablet.
(b) The elements contained in acetylsalicylic acid are hydrogen, carbon, and oxygen. The combustion of 3.000
g of the pure compound yields 1.200 g of water and 3.72 L of dry carbon dioxide, measured at 750. mm Hg
and 25C. Calculate the mass, in g, of each element in the 3.000 g sample.
(c) A student dissolved 1.625 g of pure acetylsalicylic acid in distilled water and titrated the resulting solution
to the equivalence point using 88.43 mL of 0.102 M NaOH
. Assuming that acetylsalicylic acid has only
(aq)
one ionizable hydrogen, calculate the molar mass of the acid.
(d) A 2.00  10
-3
mole sample of pure acetylsalicylic acid was dissolved in 15.00 mL of water and then titrated
with 0.100 M NaOH
. The equivalence point was reached after 20.00 mL of the NaOH solution had been
(aq)
added. Using the data from the titration, shown in the table below, determine
(i) the value of the acid dissociation constant, K
, for acetylsalicylic acid and
a
(ii) the pH of the solution after a total volume of 25.00 mL of the NaOH solution had been added (assume
Volume of
pH
0.100M NaOH
0.00
2.22
5.00
2.97
10.00
3.44
15.00
3.92
20.00
8.13
25.00
?
2000 B
O
and its hydrate.
(s)
2
4
(a) Calculate the mass percent of carbon in the hydrated form of the solid that has the formula BeC
O
3H
O
.
2
4
2
(b) When heated to 220.C, BeC
O
3H
O
dehydrates completely as represented below.
(s)
2
4
2