How much will the pH change?

A beaker with 185 mL of an acetic acid buffer with a pH of 5.00 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.100 M. A student adds 8.30 mL of a 0.300 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.760.How much will the pH change?
5.00 - 4.760 = 0.24



10^0.24 =1.74 = [acetate]/ [acetic acid]



[acetate]= 1.74 [acetic acid]



because

[acetate] + [acetic acid]= 0.100

[acetate]= 0.100 - [acetic acid]



0.100 - [acetic acid] = 1.74 [acetic acid]

[acetic acid]= 0.0365 M

[acetate]= 0.100 - 0.0365 = 0.0635 M



moles acetate = 0.185 L x 0.0635 = 0.0117

moles acetic acid = 0.185 L x 0.0365 =0.00675



moles HCl added = 0.00830 L x 0.300 M=0.00249



CH3COO- + H+ = CH3COOH

moles acetic acid = 0.00675 + 0.00249=0.00924

moles acetate = 0.0117 - 0.00249=0.00921



pH = 4.760 + log 0.00921/ 0.00924 = 4.76



delta pH =5.00 - 4.76 = 0.24