Question

Calculate the pH of the solution when the following substances are added together:   

1. 200 mL of 0.01M HCl and 8 mL 0.25M NaOH

2. 200 mL of 0.01M Acetic Acid and 80 mL of 2.5M Sodium Acetate

Answer 1

1) HCl + NaOH ----------> NaCl + H2O

millimoles of HCl = 200 x 0.01 = 2.0

millimoles of NaOH = 8.0 x 0.25 = 2.0

means equivalence point

as NaOH and HCL are strong base and strong acid at equivalence point ph = 7.0

pH = 7.0

2)  mixture of acetic acid and sodium acetate act as acidic buffer.

[CH3COOH] = (200 x 0.01 / 200 + 80) = 0.00714M

[CH3COONa] = (80 x 2.5 / 80 + 200) = 0.714 M

pKa of CH3COOH = 4.74 standard value

pH = pKa + log [CH3COONa] / [CH3COOH]

pH = 4.74 + log [0.714 / 0.00714]

pH = 6.74