### All AP Chemistry Resources

## Example Questions

### Example Question #13 : Acid Base Reactions

A solution has a hydrogen ion concentration of . What is the pH of this solution?

**Possible Answers:**

**Correct answer:**

pH can be found by taking the negative log of the hydrogen ion concentration.

We may not know the exact value of this calculation, but we know that it will be somewhere between 5 and 6.

Only one answer choice fits this range. The answer is about 5.6.

### Example Question #14 : Acid Base Reactions

Find the pH of a 0.03M solution of nitric acid.

**Possible Answers:**

**Correct answer:**

Nitric acid is a strong acid, meaning that it will dissociate completely in water.

As a result, a 0.03M nitric acid solution will dissociate completely, resulting in a proton concentration of 0.03M.

Now that we know the proton concentration, we can use the equation for pH to find the pH value of the solution.

### Example Question #15 : Acid Base Reactions

Find the pH of a 0.1M solution of hydrogen cyanide.

**Possible Answers:**

**Correct answer:**

Hydrogen cyanide will dissociate according to the following reaction.

Hydrogen cyanide is a weak acid, meaning that it will not dissociate completely in solution. We need to use the acid dissociation constant in order to determine the concentration of protons in solution at equilibrium. We can solve for this value using an ICE table.

I. The initial concentration for the acid of 0.1M. There are no protons or cyanide ions in the solution initially, so they both have concentrations of 0.0M.

C. The concentration of both ions will increase by an unknown amount, , while the acid concentration will decrease by a concentration of .

E. Write the equilibrium expression and compare it to the dissociation constant.

The value of will be small enough that it is negligible in comparison to 0.1, and can be ignored in the denominator.

From our set up, we know that the value of is equivalent to the concentration of hydrogen ions at equilibrium. Using this value, we can calculate the pH of the solution.

### Example Question #21 : P H

0.01M sodium cyanide solution is added to a 2.0M solution of hydrogen cyanide.

The acid dissociation constant for HCN is .

Find the pH of the solution.

**Possible Answers:**

**Correct answer:**

Hydrogen cyanide will dissociate in solution based on the following reaction.

By adding sodium cyanide to the solution, we have increased the concentration of the right side of the reaction. Remembering Le Chatelier's principle, we would predict that this would push the reaction to the left, and decrease the dissociation of the acid. Sodium cyanide will dissolve completely in an aqueous solution, resulting in an initial 0.01M cyanide ion solution. Knowing this, we can set up an ICE table, and solve for the pH of the solution.

I. There is initially a 2.0M concentration of acid, and 0.01M concentration of cyanide ions. There are no protons initially in the solution.

C. Both ions will increase in concentration by a value of , while the acid's concentration will decrease by a value of .

E. Placing these values in the equilibrium expression, we can calculate the value of .

Since the value for will be much less than 0.01 or 2, we can omit the variable from these factors in the equation.

Based on our set up, this concentration is equal to the concentration of protons in solution.

Now we can use the equation for pH and the proton concentration to find the final pH of the solution.

Using the pH equation, we determine that the pH of the solution is 6.9. The solution is only slightly acidic due to the initial addition of cyanide ions.

### Example Question #22 : P H

Wine has an approximate pH of 4.0. How much more acidic is wine compared to water, which has a pH of 7.0?

**Possible Answers:**

1,000 times more acidic

100 times more acidic

30 times more acidic

3 times more acidic

**Correct answer:**

1,000 times more acidic

The pH scale is a logarithmic scale, meaning that every time the value decreases by one, there has been a ten-fold increase in the acidity of the solution. Since wine is three steps more acidic than neutral water, we can say that wine is 1000 times more acidic than water.

### Example Question #21 : P H

What is the pH of a solution of HCl with a concentration of 0.0002M?

**Possible Answers:**

**Correct answer:**

pH is a measurement of the hydrogen ion concentration in a solution:

Hydrochloric acid is monoprotic and fully dissociates in solution; thus, the concentration of acid will be equal to the concentration of protons.

Now we can use the pH equation to calculate the pH form the proton concentration.

### Example Question #21 : P H

of a solution of hydrochloric acid is treated with of a solution of sodium hydroxide. What is the pH of the resulting solution?

**Possible Answers:**

**Correct answer:**

To find the pH, we must first consider how many moles of are present initially, and how many moles of are added. All initial protons will come from the acid, while all hydroxide ions will come from the base.

These are our starting amounts, before the reaction occurs.

The reaction occurs with a one-to-one ratio, meaning that the ion present in the less amount will be the limiting reagent. In this question, the hydroxide ions are the limiting reagent. We need to determine how many moles of protons remain after all of the sodium hydroxide has reacted.

The total volume of the final solution wil be equal to the sum of the two original solutions.

We can now find the final concentration of protons after the reaction.

Use the formula for pH to find the final pH of the solution from the proton concentration.

### Example Question #1 : Calculating P H And P Oh

Sodium hydroxide is a strong base. What is the pH of a 0.02M sodium hydroxide solution?

**Possible Answers:**

**Correct answer:**

Since sodium hydroxide is a strong base, it will dissociate completely in water. This means that the concentration of the base will be equal to the concentration of hydroxide ions after the reaction runs to completion.

We can find the concentration of hydroxide ions via stoichiometry. One hydroxide ion is created from each molecule of sodium hydroxide that dissociates.

Since we have the concentration of hydroxide ions, we can solve for the pOH of the solution.

The question asks us to find the pH of the solution, so we will need to convert pOH to pH. To do so, we simply subtract the pOH from 14.

The pH of the solution is 12.3. Because sodium hydroxide is a strong base, it makes sense that the pH is above 7.

### Example Question #22 : P H

Find the pH of a 0.1M solution of the weak base dimethylamine.

**Possible Answers:**

**Correct answer:**

Since dimethylamine is a weak base, we must compare the base dissociation constant to the equilibrium expression of the reaction. The reaction is written as follows:

We can determine the values for the equilibrium expression by using an ICE table:

I. There is initially a concentration of 0.1M of dimethylamine. Neither of the ions are initially present in the solution, giving them initial concentrations of 0.0M.

C. Both of the ions (products) will increase in concentration by an unknown amount, , while the base (reactant) will decrease in concentration by the value .

E. We can use the unknown concentration changes to formulate an equation for the equilibrium conditions.

The value of will be extremely small, making a negligible impact on the 0.1M denominator. The variable can be ignored in the denominator.

From our initial set up, we know that the value of is equivalent to the concentration of hydroxide ions in solution.

Keep in mind that we are dealing with the dissociation of a *base* in this question. As a result, we can expect a pH above 7 since the overall solution will be basic. We can find the pOH of the solution by using the concentration of hydroxide ions.

Now that we know the pOH, we can find the pH by using the following equation.

The pH of the solution is 11.9.

### Example Question #26 : P H

A chemist mixes of a solution with of a solution. Assuming the two solutions are additive, what is the pH of the resulting solution?

**Possible Answers:**

**Correct answer:**

To find the pH of the solution after mixing the individual components, one first has to determine the final hydroxide concentration, [^{-}OH].

(Remember that 1 mole of Ba(OH)_{2} will give 2 moles of [^{-}OH]).

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