Q: Which equation is used to calculate the pH of a buffer solution containing a weak acid and its conjugate base?

$$pH = pK_a + \log \frac{[\text{base}]}{[\text{acid}]}$$

Question 1

Which of the following best describes the common-ion effect?

Accepted Answer

The addition of a strong electrolyte containing an ion in common with a dissolved weak electrolyte suppresses the ionization of the weak electrolyte.

Question 2

A buffer solution is prepared by mixing 0.50 mol of acetic acid ($$CH_3COOH) $$and 0.50 mol of sodium acetate ($$CH_3COONa) in 1.0 L of $$water. If $$K_a$$ for acetic acid is $$1.8 	imes 10$$^{-5}$$, what is the pH of the buffer? (Use pK_a$$ = -\log K_a$$)

Accepted Answer

$$pH = 4.74$$

Question 3

Which equation is used to calculate the pH of a buffer solution containing a weak acid and its conjugate base?

Accepted Answer

$$pH = pK_a + \log \frac{[	ext{base}]}{[	ext{acid}]}$$

Question 4

A buffer solution contains 0.75 M lactic acid ($$HC_3H_5O_3) $$and 0.75 M sodium lactate ($$C_3H_5O_3Na). If$$ $$K_a$$ for lactic acid is $$1.4 	imes 10$$^{-4}$$, what is the pH of the buffer?

Accepted Answer

$$pH = 3.85$$

Question 5

During a titration of a strong acid with a strong base, what is the pH at the equivalence point?

Accepted Answer

$$pH = 7$$

Question 6

Which of the following statements about acid-base indicators is correct?

Accepted Answer

An indicator changes color at its pKa value, which should be close to the equivalence point pH.

Question 7

A solution contains $$1.0 	ext{ M} HF$$ ($$K_a = 7.2 	imes 10$$^{-4}$$) and 2.0$$ 	ext{ M}$$ NaF. What is the effect of adding NaF to the HF solution?

Accepted Answer

The dissociation of HF decreases and the pH increases.

Question 8

Which of the following best explains why human blood must be buffered?

Accepted Answer

To maintain enzyme activity and prevent fatal changes in pH.

Question 9

What is the solubility product ($$K_{sp}) $$expression for $$Ba_3$$($$PO_4)_2$?

Accepted Answer

$$K_{sp} = [Ba$$^{2+}$$]^3[PO_4$^{3-}]^2$$

Question 10

If $$Q \u003e K_{sp}$$ for a solution containing $$Mg(OH)_2$$, what will happen?

Accepted Answer

A precipitate of $$Mg(OH)_2$$ will form.

Question 11

Which of the following will increase the solubility of $$Fe(OH)_2 in $$water?

Accepted Answer

Decreasing the pH of the solution.

Question 12

A buffer is most effective when:

Accepted Answer

The concentrations of acid and conjugate base are equal.

Question 13

Which of the following is true about the pH range of a buffer?

Accepted Answer

It is typically within $$\pm 1 pH $$unit of the buffer's $$pK_a.$$

Question 14

Which of the following best describes selective precipitation in qualitative analysis?

Accepted Answer

Ions are separated based on differences in their solubility products ($$K_{sp}).$$

Question 15

A solution contains $$0.040\ 	ext{M}\ Pb$$^{2+}$$ and 0.25$$\ 	ext{M}\ Cl^-$$. Will $$PbCl_2$$ precipitate? (K$$_{sp}($$PbCl_2) = 1.6$$ 	imes $$10^{-5})$$

Accepted Answer

Yes, because $$Q = [Pb$$^{2+}$$][Cl^-]^2 = 0.040 	imes (0.25)^2$ = 0.0025 \u003e K_{sp}$$

Question 16

Which of the following is a correct statement about the titration of a weak acid with a strong base?

Accepted Answer

The pH at the equivalence point is greater than 7.

Question 17

Which of the following best explains why the solubility of $$CaF_2$$ decreases when $$NaF is $$added to the solution?

Accepted Answer

The common-ion effect reduces the solubility by increasing the concentration of $$F^-$$ ions.

Question 18

A buffer solution is prepared by mixing 0.25 M benzoic acid ($$HC_7H_5O_2) $$with sodium benzoate. What concentration of sodium benzoate is needed to achieve a pH of 3.75? ($$K_a = 6.3 	imes 10$$^{-5}$$)

Accepted Answer

0.25 M

Aqueous Equilibria: Buffers, Titrations, and Solubility (GOB Chemistry Study Notes)

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