Question

1. You are given a solution wherein [H+] is 1/4 of the concentration of [OH-]. What is the pH of this solution?

2. What is the change in pH when a few drops of strong base containing 8 x 10-4 moles of [OH-] is added to a liter of either of the following two buffered solutions at pH 7.0

i) a 150 mM solution of acetate buffer (pKa = 5.4)

ii) a 150 mM solution of glycine buffer (pKa = 8.0)

Which solution is the more effective buffer at pH 7.0 (provide an explanation for your answer)?

Answer 1

1)   we know that   [H+][OH-] = 1.0 x 10^-14

it is given [H+] = 1/4 [OH-]

[OH-] = 4 [H+]

[H+][OH-] = 1.0 x 10^-14

4 [H+] [H+] = 1.0 x 10^-14

[H+]^2 = 1/4 x 10^-14

[H+] = 1/2 x 10^-7

[H+] = 5 x 10^-8 M

pH = -log[H+]

pH = -log(5 x 10^-8)

pH = 7.30

2)

(i) salt + acid = 150mM= 0.15 M

pH = pKa + log [salt / acid]

7 = 5.4 + log [salt / 0.15-salt]

1.6 = log [salt / 0.15-salt]

salt / 0.15-salt = 39.8

salt = 5.97 - 39.8 salt

40.8 salt = 5.97

salt = 0.146 M

salt + acid = 0.150

acid = 0.150 - 0.146

acid= 0.004 M

now on addition of 8 x 10-4 moles of base to this buffer solution acid concentration decreases and salt concentration increases

new pH

pH = pKa + log [salt + base moles / acid -base moles]

pH = 5.4 + log [0.146 + 0.0008 / 0.004- 0.0008]

pH = 7.06

change in pH = 7.06 - 7

change in pH   = 0.06------------------------------------answer

(ii) salt + acid = 150mM= 0.15 M

pH = pKa + log [salt / acid]

7 = 8.0 + log [salt / 0.15-salt]

-1 = log [salt / 0.15-salt]

salt / 0.15-salt = 0.1

salt = 0.015 - 0.1 salt

1.1 salt = 0.015

salt = 0.0136 M

salt + acid = 0.150

acid = 0.150 - 0.0136

acid= 0.136 M

now on addition of 8 x 10-4 moles of base to this buffer solution acid concentration decreases and salt concentration increases

new pH

pH = pKa + log [salt + base moles / acid -base moles]

pH = 8.0 + log [0.0136 + 0.0008 / 0.136 - 0.0008]

pH = 7.03

change in pH = 7.03 - 7

change in pH   = 0.03------------------------------------answer

pH change is less comapred to acetate buffer

effective buffer is a 150 mM solution of glycine buffer (pKa = 8.0) . because on addition of strong base the pH change is small 0.03 then first one.