Q: Which buffer system is most effective for maintaining a pH of 4.45?

HC$_2$O$_4^-$/C$_2$O$_4^{2-}$ ($K_{a2} = 5.1 \times 10^{-5}$)

Question 1

A 0.750 L solution of 0.350 M formic acid (HCOOH, $K_a = 1.8 	imes 10^{-4}$) is to be made into a buffer with a pH of 3.745 by adding sodium formate (NaCHO$_2$). What mass of NaCHO$_2$ (molar mass = 68.01 g/mol) must be added? Use the Henderson-Hasselbalch equation: $pH = pK_a + \log \left( \frac{[A^-]}{[HA]} ight)$.

Accepted Answer

4.2 g

Question 2

Which of the following 0.10 M solutions has the highest pH?

Accepted Answer

0.10 M HF(aq)

Question 3

Rank the following acids in order of decreasing acid strength in aqueous solution: HBr, HOCl, HOBr, HOI.

Accepted Answer

HBr > HOCl > HOBr > HOI

Question 4

A 0.020 M solution of a weak monoprotic acid has a pH of 3.28 at 25°C. What is the acid dissociation constant ($K_a$) for this acid? Use $[H^+] = 10^{-pH}$ and $K_a = \frac{[H^+][A^-]}{[HA]}$.

Accepted Answer

$2.6 	imes 10^{-6}$

Question 5

What is the hydronium ion concentration $[H_3O^+]$ in a 0.0041 M LiOH solution at 25°C? (Assume complete dissociation and $K_w = 1.0 	imes 10^{-14}$)

Accepted Answer

$2.4 	imes 10^{-12}$ M

Question 6

Which buffer system is most effective for maintaining a pH of 4.45?

Accepted Answer

HC$_2$O$_4^-$/C$_2$O$_4^{2-}$ ($K_{a2} = 5.1 	imes 10^{-5}$)

Question 7

A solution has a pH of 10.20 at 25°C. What is the hydroxide ion concentration $[OH^-]$ at 25°C? ($pOH = 14 - pH$ and $[OH^-] = 10^{-pOH}$)

Accepted Answer

$1.6 	imes 10^{-4}$ M

Question 8

What is the percent ionization of a 1.5 M acetic acid (HC$_2$H$_3$O$_2$) solution at 25°C? ($K_a = 1.8 	imes 10^{-5}$)

Accepted Answer

0.35%

Question 9

What is the pH of a solution that is 0.048 M in HA and 0.0032 M in NaA? ($K_a = 4.1 	imes 10^{-6}$) Use the Henderson-Hasselbalch equation.

Accepted Answer

4.07

Question 10

Which of the following solutions is basic at 25°C? Use the provided $K_a$ and $K_b$ values for relevant ions.

Accepted Answer

Sodium acetate (NaC$_2$H$_3$O$_2$)

Question 11

A 225 mL buffer solution is 0.485 M in HCN and 0.385 M in LiCN. Calculate the initial pH after the addition of 0.0350 moles of NaOH. ($K_a$ for HCN = $6.2 	imes 10^{-10}$). Use the Henderson-Hasselbalch equation.

Accepted Answer

9.72

Question 12

A 225 mL buffer solution is 0.485 M in HCN and 0.385 M in LiCN. What is the pH after the addition of 0.0225 moles of HCl? ($K_a$ for HCN = $6.2 	imes 10^{-10}$).

Accepted Answer

8.97

Question 13

A student prepares a buffer by mixing 0.20 mol of a weak acid (HA, $K_a = 1.0 	imes 10^{-5}$) and 0.10 mol of its conjugate base (A$^-$) in 1.0 L of water. What is the pH of the buffer? Use the Henderson-Hasselbalch equation.

Accepted Answer

4.40

Question 14

If 0.10 mol of HCl is added to 1.0 L of the buffer in the previous question, what will be the new pH? Assume the volume does not change.

Accepted Answer

4.00

Question 15

Which of the following statements about buffer solutions is correct?

Accepted Answer

A buffer is most effective when $[HA] = [A^-]$ and $pH = pK_a$

Question 16

A solution is prepared by dissolving 0.10 mol of NH$_4$Cl and 0.10 mol of NH$_3$ in enough water to make 1.0 L. ($K_b$ for NH$_3$ = $1.8 	imes 10^{-5}$). What is the pH of the solution?

Accepted Answer

9.25

Question 17

Which of the following combinations would NOT result in a buffer solution?

Accepted Answer

0.10 mol HCl + 0.10 mol NaCl

Question 18

A 0.10 M solution of a weak acid has a pH of 3.00. What is the percent ionization of the acid?

Accepted Answer

1.0%

Question 19

Which of the following best explains why HF is a weaker acid than HCl, even though both are hydrogen halides?

Accepted Answer

The H–F bond is much stronger than the H–Cl bond, making HF less likely to ionize.

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