pH & pOH (College Board AP® Chemistry)

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.

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.

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

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

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.

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.

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)            K_b = = 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.)

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.

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.