Question

QUESTION 1

Which pair of compounds will form a buffer in aqueous solution?

	a.	HCN and NaCN
	b.	NaCN and NaOH
	c.	HCl and NaOH
	d.	HCl and NaCl
	e.	HCN and HCl
	f.	NaCN and KCN

10 points   

QUESTION 2

A buffer solution is titrated with 0.032 M HCl solution. The initial volume of the buffer is 25.00 mL. The equivalence point of the titration is determined to be 4.88 mL. What is the concentration of conjugate base (in M) in the buffer solution? Assume the stoichiometry of acid to base is 1:1.

10 points   

QUESTION 3

A buffer solution is titrated with 0.046 M NaOH solution. The initial volume of the buffer is 25.00 mL. The equivalence point of the titration is determined to be 8.77 mL. What is the concentration of acid (in M) in the buffer solution? Assume the stoichiometry of acid to base is 1:1.

10 points   

QUESTION 4

The concentrations of acid and conjugate base in a buffer are determined experimentally to be 0.044 M and 0.059 M, respectively. If the total buffer concentration given on the bottle (the true value) is 0.102 M, then what is the percent error of the experimental value?

10 points   

QUESTION 5

There are several calculations that need to be done in order to prepare a specific volume of buffer solution at a particular concentration and pH. There is a worked example in your lab manual on p. 119.

First, determine the correct A^-/HA ratio:  The pK_a of acetic acid is 4.76, and the buffer solution needs to maintain a pH of 4.25. The concentration of acetate (CH₃CO₂^-) should be X times that of the concentration of acetic acid (CH₃CO₂H). What is X?

10 points   

QUESTION 6

There are several calculations that need to be done in order to prepare a specific volume of buffer solution at a particular concentration and pH. There is a worked example in your lab manual on p. 119.

Once the correct ratio of A^- to HA is known, determine the actual concentration of A- needed to make a buffer at a particular concentration.

The ratio of acetate to acetic acid in a buffer is 0.176. That is, the concentration of acetate should be 0.176 times that of the concentration of acetic acid. What is the concentration of acetate (in M) needed to prepare a 0.416 M buffer using acetic acid and acetate?

10 points   

QUESTION 7

There are several calculations that need to be done in order to prepare a specific volume of buffer solution at a particular concentration and pH. There is a worked example in your lab manual on p. 119.

What volume of acetate solution (in mL) is needed to make 250.0 mL of a buffer solution? The concentration of acetate in the buffer must be 0.0594 M, and the concentration of the acetate stock solution is 0.500 M.

Answer 1

1. A buffer is a solution which resists a change in pH on addition of small amounts of acid/base. There are two types of buffer solutions

i. Acidic buffer- A buffer containing a weak acid and its conjugate base.

ii. Basic buffer - A buffer containing a weak base and its conjugate acid.

a. HCN is a weak acid and NaCN is a salt containing CN^-, the conjugate base of HCN. So this pair forms a buffer solution. So correct answer is option a.

b. NaOH is a strong base whereas NaCN is a salt containing CN^- (conjugate base of weak acid HCN). So this pair doesn't form a buffer.

c. HCl is a strong acid and NaOH is a strong base. So this pair can't form a buffer.

d. HCl is a strong acid whereas NaCl is salt of its conjugate base Cl^-. But it doesn't fall in the definition of buffer. So this pair doesn't form a buffer.

e. HCN is a weak acid and HCl is a strong acid. So this pair can't form a buffer as we don't have the conjugate base of the weak acid in this pair.

f. Both NaCN and KCN are salts containing CN^- which is the conjugate base of weak acid HCN. But we don't have the weak acid HCN in this pair. So this pair doesn't form a buffer.

So answer is option a.