pH & pOH (College Board AP® Chemistry): Exam Questions

38 mins15 questions
1a1 mark

A student is analyzing the pH of a solution and finds that the pOH is 4.25 at 25°C.

Using the ionization constant of water (Kw), determine the pH of the solution and explain why pH + pOH = 14 at this temperature.

1b2 marks

i) Calculate the pH of the solution using your answer from part (a). Show all working.

ii) Identify whether the solution is acidic, basic, or neutral based on your answer from part (i). Justify your answer by comparing the pH to 7.

1c1 mark

If the temperature increases above 25°C, predict how the pH of pure water will change. Justify your answer.

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2a1 mark

A 0.0040 M solution of sodium hydroxide, NaOH, is prepared in water at 25°C.

Write a balanced chemical equation that represents the dissociation of sodium hydroxide in water.

2b1 mark

Calculate the concentration of hydroxide ions (OH) in the solution. Explain how this relates to the dissociation of strong bases.

2c1 mark

i) Determine the pOH of the solution. Show all working.

ii) Using your answer from part (i), calculate the pH of the solution at 25°C. Justify your answer using the relationship between pH and pOH.

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3a1 mark

At 50 °C, the ionization constant of water (Kw) is 5.47 × 10−14.

Write the balanced chemical equation for the autoionization of water.

3b1 mark

Determine the pH of pure water at 50 °C. Show all working.

3c1 mark

Explain how an increase in temperature affects the value of Kw and the pH of pure water. Justify your answer based on the enthalpy change of the autoionization of water

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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) → 2NaNO3 (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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2a1 mark

A 0.0200 M solution of barium hydroxide (Ba(OH)2 is prepared at 25°C.

Write the dissociation equation for Ba(OH)2 in water and explain how it differs from monoprotic bases.

2b1 mark

Calculate the hydroxide ion concentration in the solution. Justify your calculation based on stoichiometry.

2c1 mark

Determine the pH of the solution at 25 °C. Show all work with correct significant figures.

2d1 mark

If the concentration of Ba(OH)2 is doubled, determine the effect on the pH of the solution. Justify your answer with appropriate calculations. Show all work.

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3a2 marks

A 50.0 mL solution of 0.100 M HCl is mixed with 25.0 mL of 0.200 M NaOH at 25°C.

Determine whether the final solution is acidic, basic, or neutral. Show all calculations to justify your answer.

3b2 marks

A student prepares two solutions:

Solution A: A neutral solution at 25°C

Solution B: A neutral solution at 50°C

The student claims that both solutions have the same pH. Evaluate this claim and justify your answer using the relationship between pH, pOH, and Kw.

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4a1 mark

At 25 °C, the ionization constant of water, Kw​, is 1.00×10−14. At 50 °C, the value of Kw increases to 5.47 x 10−14.

Calculate the pH and pOH of pure water at 50°C. Show all working.

4b1 mark

A student claims that since pH is lower at higher temperatures, water becomes acidic. Evaluate this claim and justify your answer using equilibrium principles and the relationship between pH and pOH.

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