Question

1. A buffer solution contains 0.378 M KHCO3 and 0.268 M K2CO3. Determine the pH change when 0.094 mol KOH is added to 1.00 L of the buffer.

pH after addition − pH before addition = pH change =

2. Determine the pH change when 0.066 mol HI is added to 1.00 L of a buffer solution that is 0.345 M in CH₃COOH and 0.262M in CH₃COO^-.

pH after addition − pH before addition = pH change =

Answer 1

1) pKa of HCO3- = 10.32

The buffer is

HCO3- + OH- --------------------> CO3-2 + H2O

0.378 - 0.268 - initial moles

Ph of buffer =  pKa + log [conjugate base]/[acid]

= 10.32 + log 0.268/0.378

= 10.17

- 0.094 - - cahnge

0.284 0 0.362 - at equilibrium

Thus the pH after addition as calculated using Hendersen equation

pH = pKa + log [conjugate base]/[acid]

= 10.32 + log 0.362/0.284

=10.4253

Thus the pH change = 10.4253 -10.170

= 0.2553

2) pKa of acetic acid = 4.75

the buffer is

CH3COOH ---------------->CH3COO- + H+

0.345 0.262 0

pH before addition = 4.75 + log 0.262/0.345

= 4.63

After addition of 0.066M HI

CH3COOH ---------------->CH3COO- + H+

0.345 0.262 0 initial moles

- - 0.066 change

0.411 0.196 0 after addition

Thus the pH of buffer = 4.75 + log 0.196/0.411

= 4.428

Thus the change in pH = 4.428 - 4.63

= - 0.202