5 Steps to a 5 AP Chemistry 2019

AP Chemistry Practice Exam 1 ❮ 331

Question 3

A student wishes to analyze a sample of ammonium sulfate, (NH 4 ) 2 SO 4 , contaminated with sodium sul-

fate, Na 2 SO 4. She constructs the apparatus shown above to carry out this analysis. She weighs 1.002 grams

of sample into flask A and dissolves the solid in 50 mL of water. Then she adds 50.00 mL of 0.4000 M

hydrochloric acid, HCl, to flask B. In the next step, she quickly adds an excess of concentrated sodium

hydroxide, NaOH, solution to flask A and quickly seals the system. She then heats flask A to boiling and

distills over about 25 mL of water to flask B; during this process all the ammonia, NH 3 , generated in

flask A transfers to flask B. After the distillation is complete, she disassembles the apparatus and adds a

small amount of methyl red indicator to flask B and titrates the solution in flask B with standard sodium

hydroxide solution. The titration requires 35.25 mL of 0.1600 M sodium hydroxide solution to reach the

endpoint.

The reactions are:

(1) (NH 4 ) 2 SO 4 (aq) + 2 NaOH(aq) → Na 2 SO 4 (aq) + 2 NH 3 (aq) + 2 H 2 O(l)

(2) NH 3 (aq) + HCl(aq) → NH 4 Cl(aq)

(3) NaOH(aq) + HCl(aq) → NaCl(aq) + H 2 O(l)

The first equation is for a weak acid (pKa NH 4 + = 9.25)/strong base reaction. The second equation is for

a weak base/strong acid reaction. The third equation is for a strong base/strong acid reaction.

Information on indicators:

pH Range pKa Color Change

Methyl red 4.2–6.2 5.0 Red → Yellow

Phenolphthalein 8.2–10.0 9.3 Colorless → Pink

(a) (i) Calculate the moles of hydrochloric acid originally in flask B.

(ii) Calculate the moles of hydrochloric acid reacting with the sodium hydroxide solution.

(iii) Calculate the moles of ammonia that reacted with the hydrochloric acid.

(b) (i) Calculate the mass of ammonium sulfate in the sample. (Molar mass of ammonium sulfate =

132.139 g)

(ii) Calculate the percent ammonium sulfate in the sample.

(c) (i) Strong acid strong base titrations commonly use phenolphthalein as an indicator. Give a good

reason why the student chose to use methyl red instead.

(ii) A second student used phenolphthalein in place of methyl red and got significantly different

results. Was the second student’s percent higher or lower than the student using methyl red?

What was the cause of this discrepancy?

(d) In the box below, sketch the Lewis electron-dot structure of ammonium chloride.

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