Q: Which shortcut can be used to estimate $$[H^+]$$ for a weak acid when $$K_a is $$very small and the initial concentration is much larger than $$K_a$$?

$$[H^+] = \sqrt{K_a \times [HA]_{initial}}$$

Q: If the pKa of a weak acid is 5.57, what is the value of $$K_a$$?

$$K_a = 2.69 \times 10$$^{-6}$$

Q: Which of the following is the correct formula for calculating percent ionization of a weak acid?

$$\% \text{ ionization} = \dfrac{[H^+]_{eq}}{[HA]_{initial}} \times 100$$

Question 1

Which of the following statements best describes a weak acid in aqueous solution?

Accepted Answer

A weak acid only partially dissociates into ions in water.

Question 2

Given a 0.30 M solution of HCN with $$K_a = 4.9 	imes 10$$^{-10}$$, which of the following is the correct setup for the equilibrium expression?

Accepted Answer

$$K_a = \dfrac{[H^+][CN^-]}{[HCN]}$$

Question 3

What is the first step when using an ICE chart to solve for the pH of a weak acid solution?

Accepted Answer

Write the balanced chemical equation for the dissociation of the acid.

Question 4

A 0.174 M solution of nitrous acid ($$HNO_2) $$has a $$K_a of$$ $$4.4 	imes 10$$^{-4}$$. Which formula would you use to solve for $$[H^+]$$ if the ICE chart leads to a quadratic equation?

Accepted Answer

$$x = \dfrac{-b \pm \sqrt{b^2$ - 4ac}}{2a}$$

Question 5

Which of the following best explains why the percent ionization of a weak acid increases as the initial concentration decreases?

Accepted Answer

The equilibrium shifts to produce more ions at lower concentrations.

Question 6

Calculate the percent ionization for a 0.10 M solution of acetic acid ($$K_a = 1.8 	imes 10$$^{-5}$$) if $$[H^+]$$ at equilibrium is found to be 1.34$$ 	imes $$10^{-3} M.$$

Accepted Answer

$$13.4\%$$

Question 7

Which shortcut can be used to estimate $$[H^+]$$ for a weak acid when $$K_a is $$very small and the initial concentration is much larger than $$K_a$$?

Accepted Answer

$$[H^+] = \sqrt{K_a 	imes [HA]_{initial}}$$

Question 8

A student is asked to calculate the pH of a 0.50 M solution of methylammonium bromide ($$CH_3NH_3Br$$), given $$K_a$$ for methylamine is $$4.4 	imes 10$$^{-4}$$. What is the correct approach?

Accepted Answer

Use the $$K_a of $$the conjugate acid ($$CH_3NH_3^+) to $$set up the ICE chart for the acid.

Question 9

If the pKa of a weak acid is 5.57, what is the value of $$K_a$$?

Accepted Answer

$$K_a = 2.69 	imes 10$$^{-6}$$

Question 10

A weak acid solution has an initial concentration of 0.35 M and a pKa of 5.57. What is the pH of the solution?

Accepted Answer

pH = 6.34

Question 11

Which of the following is the correct formula for calculating percent ionization of a weak acid?

Accepted Answer

$$\% 	ext{ ionization} = \dfrac{[H^+]_{eq}}{[HA]_{initial}} 	imes 100$$

Question 12

Why is it important to check if a shortcut can be used instead of the quadratic formula when solving weak acid equilibrium problems?

Accepted Answer

Shortcuts can simplify calculations when $$x is $$much smaller than the initial concentration.

Question 13

Which step in the ICE chart method involves substituting equilibrium concentrations into the $$K_a$$ expression?

Accepted Answer

Equilibrium row

Question 14

A solution is prepared by dissolving 0.10 mol of a weak acid in enough water to make 1.0 L of solution. If the $$K_a is$$ $$1.8 	imes 10$$^{-5}$$, what is the approximate $$[H^+]$$ using the shortcut formula?

Accepted Answer

$$[H^+] = \sqrt{1.8 	imes 10$$^{-5}$$ 	imes 0.10}$$

Question 15

Which of the following best describes the relationship between $$K_a$$ and acid strength?

Accepted Answer

A larger $$K_a$$ value indicates a stronger acid.

Question 16

If the $$[H^+] in a $$solution is $$2.5 	imes 10$$^{-4}$$ M, what is the pH?

Accepted Answer

pH = 3.60

Question 17

A student calculates a negative value for $$x$$ when solving the quadratic formula for a weak acid equilibrium. What should they do?

Accepted Answer

Ignore the negative value and use the positive root.

Question 18

Which of the following is the correct formula for calculating pH from $$[H^+]$$?

Accepted Answer

$$	ext{pH} = -\log [H^+]$$

pH of Weak Acids and ICE Chart Calculations

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