5 Steps to a 5 AP Chemistry

 Free-Response Questions

First Free-Response Question

You have 20 minutes to do the following question. You may use a calculator and the tables

at the back of the book.

A hydrogen gas sample is collected over water. The volume of the sample was 190.0 mL

at 26°C, and the pressure in the room was 754 mm Hg. The vapor pressure of water at

26 °C is 25.2 mm Hg.

a. Calculate the number of moles of hydrogen in the sample.

b. Calculate how many molecules of water vapor are present in the sample.

c. Determine the density (in g/L) of the gas mixture.

d. Determine the mole fraction of water.

Second Free-Response Question

You have 20 minutes to do the following questions. You may not use a calculator. You may

use the tables in the back of the book.

A sample containing 2/3 mole of potassium chlorate, KClO 3 , is heated until it

decomposes to potassium chloride and oxygen gas. The oxygen is collected in an inverted

bottle through the displacement of water. Answer the following questions using this

information.

a. Write a balanced chemical equation for the reaction.

b. How many moles of oxygen gas are produced?

c. The temperature and pressure of the sample are adjusted to STP. The volume of the

sample is found to be slightly greater than 22.4 liters. Explain.

d. An excess of sulfur is burned in the oxygen. Write a balanced chemical equation

and calculate the number of moles of gas formed.

e. After the sulfur had completely reacted, a sample of the residual water was removed

from the bottle and found to be acidic. Explain.

 Answers and Explanations

First Free-Response Question

a. Ptotal=754 mm Hg Phydrogen=754 – 25.2 =729 mm Hg

P= 729 mm Hg/760 mm Hg =0.959 atm

V=190.0 mL =0.1900 L T= 26 °C =299 K

R=0.0821 L atm mol−^1 K−^1

n=PV/RT

=(0.959 atm ×0.1900 L)/(0.0821 L atm mol−^1 K−^1 ×299 K)

=0.00742 mol H 2

Give yourself 1 point for the correct answer (no deduction for rounding

differently). You must include allparts of the calculation (including “=”).

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