Find the pKa for each acid in the conjugate acid–base pairs, given the Ka value.

ID: 505867 • Letter: F

Question

Acid–Base Pair

A CH3CO2H and NaC2H3O2

B NH4Cl and NH3

Ka pKa 1.8 105
5.6 1010

Suppose you needed to make a buffer of pH 5.15.

a) Which buffer system (from Question 1) would be the better choice?

b) Calculate the ratio of conjugate base to conjugate acid (i.e. [A]) needed to make the buffer with the

buffer system you chose in (a).

2. Suppose you needed to make a buffer of pH 5.15.
a) Which buffer system (from Question 1) would be the better choice?

c) Based on your [A] value, which component of the buffer system is more concentrated: the conjugate [HA]

acid, or the conjugate base? (Write its formula.)

d) Calculate the concentration of the less concentrated component, assuming the more concentrated com- ponent (determined in (c)) is 0.30 M.

e) How much acid do you need to make 250 mL of the buffer?

f) How much base do you need to make 250mL of the buffer?

Find the pKa for each acid in the conjugate acid–base pairs, given the Ka value.

Acid–Base Pair

A) CH3CO2H and NaC2H3O2

Ka = 1.8 x 10^-5

pKa = -log Ka = -log (1.8 x 10^-5)

pKa = 4.74

B) NH4Cl and NH3

Ka = 5.6 x 10^-10

pKa = -log ( 5.6 x 10^-10)

pKa = 9.25

Suppose you needed to make a buffer of pH 5.15.

a) Which buffer system (from Question 1) would be the better choice?

CH3COOH / NaCH3COO is better choice

because . the pH value is close to pKa value of that buffer system

b) Calculate the ratio of conjugate base to conjugate acid (i.e. [A]) needed to make the buffer with the

buffer system you chose in (a).

pH = pKa + log [CH3COO- / CH3COOH]

5.15 = 4.75 + log [CH3COO- / CH3COOH]

[CH3COO- / CH3COOH] = 2.51

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