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Gases Free-Response Questions for AP Chemistry

based on 2 ratings
By — McGraw-Hill Professional
Updated on Feb 9, 2011

Review the following concepts if necessary:

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. Ptotal = 754 mm Hg                     Phydrogen = 754 – 25.2 = 729 mm Hg
  2. 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.

  3. Pwater = 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
  4. 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.

  5. (0.00742 mol H2) × (2.016 g H2/mol H2) = 0.0150 g H2
  6. (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.

  7. 0.00742 mol H2 and 2.57 × 10–4 mol H2O
  8. 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 ad. 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)
  2. You get 1 point if you have the above equation.

  3. (2/3 mole KClO3) (3 moles O2/2 moles KClO3) = 1 mole O2
  4. 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.

  5. 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.
  6. You get 1 point for discussing STP and 22.4 liters, and 1 point for discussing the presence of water vapor.

  7. 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)
  8. 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.

  9. 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
  10. Total your points for the different parts. There are 8 points possible.

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