Gases Free-Response Questions for AP Chemistry

Review the following concepts if necessary:

• Kinetic Molecular Theory for AP Chemistry
• Gas Law Relationships for AP Chemistry
• Boyle's, Charles's, Gay-Lusaac's and Avogadro's Gas Law for AP Chemistry

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.

1. Calculate the number of moles of hydrogen in the sample.
2. Calculate how many molecules of water vapor are present in the sample.
3. Determine the density (in g/L) of the gas mixture.
4. 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, KClO₃, 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.

1. Write a balanced chemical equation for the reaction.
2. How many moles of oxygen gas are produced?
3. 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.
4. An excess of sulfur is burned in the oxygen. Write a balanced chemical equation and calculate the number of moles of gas formed.
5. 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

1. P_total = 754 mm Hg                     P_hydrogen = 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 H₂

Give yourself 1 point for the correct answer (no deduction for rounding differently). You must include all parts of the calculation (including "=").

Give yourself 1 point for the correct equation, or for any other correct calculation. Do not give yourself more than 2 points total for this part.

2. P_water = 25.2 mm Hg/760 mm Hg = 0.0332 atm. T and V are the same as in part a. Avogadro's number = 6.022 × 10²³ molecules mol^–1 = N

n = PV/RT

  = (0.0332 atm × 0.1900 L)/(0.0821 L atm mol^–1 K^–1 × 299 K)

  = 2.57 × 10^–4 mol H₂O)

molecules = (2.57 × 10^–4 mol) × (6.022 × 10²³ molecules mol^–1)

        = 1.55 × 10²⁰ molecules

Give yourself 1 point for the correct answer (no deduction for rounding differently). You must include all parts of the calculation (including "=").

Give yourself 1 point for the correct equation, or for any other correct calculation.

Do not give yourself more than 2 points total for this part.

3. (0.00742 mol H²) × (2.016 g H²/mol H²) = 0.0150 g H²

(2.57 × 10^–4 mol H²O) × (18.02 g H²O/mol H²O) = 0.00463 g H²O

total mass = 0.0150 g H² + 0.00463 g H²O = 0.0196 g

density = 0.0196 g/0.1900 L = 0.103 g/L

Give yourself 1 point for the correct answer (no deduction for rounding differently). You must include all parts of the calculation (including "=").

Give yourself 1 point for the correct equation, or for any other correct calculation.

Do not give yourself more than 2 points total for this part.

4. 0.00742 mol H² and 2.57 × 10^–4 mol H2O

total moles = (0.00742 + 2.57 × 10^–4) = 0.00768 mol

Mole fraction H²O = (2.57 × 10^–4 mol H²O)/(0.00768 mol)

= 0.0335

Give yourself 1 point for the correct answer (no deduction for rounding differently). You must include all parts of the calculation (including "=").

Give yourself 1 point for the correct equation, or for any other correct calculation.

Do not give yourself more than 2 points total for this part.

Total score = sum of parts a–d. If any of the final answers has the incorrect number of significant figures, subtract one point.

Second Free-Response Question

1. 2 KClO₃(s) → 2 KCl(s) + 3 O₂(g)

You get 1 point if you have the above equation.

2. (2/3 mole KClO₃) (3 moles O₂/2 moles KClO₃) = 1 mole O₂

You get 1 point for the correct answer and 1 point for the work. You can get these points if you correctly use information from an incorrect equation in part a.

3. At STP the volume of 1 mole of O₂ should be 22.4 liters. The volume is greater because oxygen was not the only gas in the sample. Water vapor was present. The presence of the additional gas leads to a larger volume.

You get 1 point for discussing STP and 22.4 liters, and 1 point for discussing the presence of water vapor.

4. There are two acceptable equations; either will get you 1 point. You do not need both equations.
   S(s) + O₂(g)→ SO₂(g)
         2 S(s) + 3 O₂(g) → 2 SO₃(g)

If you chose the first equation, the moles of gas produced would be:

(1 mole O₂) (1 mole SO₂/1 mole O₂) = 1 mole SO₂

If you chose the second equation, the moles of gas produced would be:

(1 mole O₂) (2 mole SO₃/3 mole O₂) = 2/3 mole SO₂

You get 1 point for either of these solutions. You will also get 1 point if you used an incorrect number of moles of O₂ from a wrong answer for part b.

5. Either of the possible sulfur oxides will dissolve in water to produce an acid. This will get you 1 point, as will a similar comment and either of the following equations:
   SO₂ + H₂O → H₂SO₃
   SO₃ + H₂O → H₂SO₄

Total your points for the different parts. There are 8 points possible.