pH & pOH (College Board AP® Chemistry)

Exam Questions

15 mins6 questions
1a1 mark

A student is given 50.0 mL of a solution of Na2CO3(aq) of unknown concentration. To determine the concentration of the solution, the student mixes the solution with excess 1.0 M Ca(NO3)2(aq) , causing a precipitate to form. The balanced equation for the reaction is shown below.

Na2CO3(aq) + Ca(NO3)2(aq) → 2 NaNO3(aq) + CaCO3(s)

Write the net ionic equation for the reaction that occurs when the solutions of Na2CO3 and Ca(NO3)2 are mixed.

1b1 mark

The diagram below is incomplete. Draw in the species needed to accurately represent the major ionic species remaining in the solution after the reaction has been completed.

q3b-june-2019---ap-chemistry
1c1 mark

The student filters and dries the precipitate of CaCO3 (molar mass 100.1 g/mol) and records the data in the table below.

Volume of Na2CO3 solution

50.0 mL

Volume of 1.0 M Ca(NO3)2 added

100.0 mL

Mass of CaCO3 precipitate collected

0.93 g

Determine the number of moles of Na2CO3 in the original 50.0 mL of solution.

1d1 mark

The student realizes that the precipitate was not completely dried and claims that as a result, the calculated Na2CO3 molarity is too low. Do you agree with the student’s claim? Justify your answer.

1e1 mark

After the precipitate forms and is filtered, the liquid that passed through the filter is tested to see if it can conduct electricity. What would be observed? Justify your answer.

1f3 marks

The student decides to determine the molarity of the same Na2CO3 solution using a second method. When Na2CO3 is dissolved in water, CO32−(aq) hydrolyzes to form HCO3  (aq), as shown by the following equation.

 CO32−(aq) + H2O(l)  → HCO3(aq) + OH (aq)            Kb = fraction numerator left square bracket H C O subscript 3 to the power of minus right square bracket left square bracket O H to the power of minus space right square bracket over denominator left square bracket C O subscript 3 to the power of 2 minus end exponent space right square bracket end fraction = 2.1 × 10−4

The student decides to first determine [OH] in the solution, then use that result to calculate the initial concentration of CO32−(aq).

i) Identify a laboratory method (not titration) that the student could use to collect data to determine [OH] in the solution.

 

ii) Explain how the student could use the measured value in part (f)(i) to calculate the initial concentration of CO32−(aq). (Do not do any numerical calculations.)

1g1 mark

In the original Na2CO3 solution at equilibrium, is the concentration of HCO3(aq) greater than, less than, or equal to the concentration of CO32−(aq) ? Justify your answer.

1h1 mark

The student needs to make a CO32− / HCO3 buffer. Is the Na2CO3 solution suitable for making a buffer with a pH of 6 ? Explain why or why not.

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