Calculate the change in pH if 10mL of 0.1M HCl is added to the buffer made by mixing 25 mL of 1.0 M CH3COOH and 25 mL of 0.5 M CH3COONa.

Answer 1

First, determine the number of moles for CH3COOH (HAc), CH3COONa (Ac^-), and HCl

Molarity=moles/Volume (L)

Moles=molarity*volume (L)

Moles of HCl=0.1M*0.010L=x
Moles of HAc=1.0M*0.025L=y
Moles of Ac^-=0.5M*0.025L=z

Calculate the pH of the buffer:

pH=pka+log[A^-/HA]

Where

pka=4.75
A^-=z
HA=y
and
pH=???

Solve for pH:

pH=4.75+log[z/y]

When HCl is added to the solution, HAc will increase and Ac^- will decrease.

Solve for the amount of change:

y+x=y1

z-x=z1

Use the Henderson-Hasselbalch equation and solve for the pH:

pH=4.75+log[y1/z1]

Subtract the pH of the buffer after addition of HCl from the pH of the buffer after addition of HCl.

Answer 2

pH=4.66