AP Chemistry : pH

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #31 : Acid Base Reactions

Find the pH of a 0.2M calcium hydroxide solution. 

Possible Answers:

Correct answer:

Explanation:

Since every calcium hydroxide molecule will dissociate and form two hydroxide ions, the concentration of the hydroxide ions will be twice as much as the initial concentration of the base.

This means that the concentration of hydroxide ions in the solution will be 0.4M.We can use this concentration to solve for the pOH fo the solution.

Since we are looking for the pH of the solution, we simply subtract the pOH form 14.

This results in a pH of 13.6.

Example Question #81 : Ap Chemistry

Which of the following is a possible pH value for a basic solution?

Possible Answers:

None of these

Correct answer:

Explanation:

Basic solutions have a pH level greater than 7, which is the neutral pH. Acidic solutions have a pH level lower than 7.

Example Question #31 : Acid Base Reactions

Which of the following is considered a neutral pH?

Possible Answers:

Correct answer:

Explanation:

Basic solutions have a pH level greater than 7 and acidic solutions have a pH level lower than 7. A pH of 7 corresponds to a neutral solution.

Example Question #31 : Acid Base Reactions

A chemist has an unknown solution. He think the solution is an acid. Which of the following pH's would support his hypothesis?

Possible Answers:

None of these

Correct answer:

Explanation:

Acidic solutions have a pH level lower than 7, which is the neutral pH. Basic solutions have a pH level greater than 7.

Example Question #31 : P H

Knowing that the acid ionization constants of  are  and , a solution of will:

Possible Answers:

have an acidic pH

have a pH greater than 7

exhibit a neutral pH because  is a salt

None of the other answers

Correct answer:

have a pH greater than 7

Explanation:

Potassium sulfide is soluble in water yielding the specie  which undergoes a two steps basic hydrolysis:

The respective hydrolysis constants are:

Analyzing the rounded values of both hydrolysis constants, we can see that the value of  is very large then we can assume the first reaction occurs to completion and controls the concentration of  in the solution which will be . Hence, the pH will be:

 

Example Question #35 : Reactions And Equilibrium

Consider the following reaction of acetic acid:

The pKa for this reaction is . At what pH would the concentrations of the acidic and basic forms of acetic acid be equal to each other?

 

Possible Answers:

Correct answer:

Explanation:

This question is presenting us with a reversible chemical reaction of a weak acid. It provides us with the pKa of the acid, and asks us to determine where on the pH spectrum will the acidic form of acetic acid be equal to its basic form.

There is a very important concept that this question highlights, which is that a compound will always have equal amounts of its acidic and basic forms at a pH that is equal to its pKa. This can be shown mathematically by using the Henderson-Hasselbalch equation.

As the above equation shows, when the acidic and basic forms are equal, their ratio is equal to . And the logarithm of  is equal to . Thus, what we're left with is:

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