Determine the pH of a buffer solution of MOPSO-H and MOPSO.

Question

1. Determine the pH of the buffer solution that is prepared by mixing 925 mL of 0.812 M MOPSO−H (C₇H₁₅NO₅S ) to 856 mL of 0.340 M MOPSO (C₇H₁₄NO₅S⁻ ). Assume the 5% approximation is valid. The pKa of MOPSO−H is 6.90.

2. Determine the pH of the MOPSO−H/MOPSO buffer solution after the addition of 358 mL of 0.178 M solution of potassium hydroxide to the existing buffer solution. Assume the 5% approximation is valid.

Answer 1

1). Use the Henderson Hasselbalch equation: pH = pKa + log [conj.base] / [weak acid]

The weak acid is the MOPSO-H (which we shall abbreviate as HA).

The conjugate base is the MOPSO- (which we shall abbreviate as A-).

moles HA = 925 ml x 1 L / 1000 ml x 0.812 mol / L = 0.7511 moles

moles A- = 856 ml x 1 L / 1000 ml x 0.340 mol / L = 0.2910 moles

Final volume of solution = 925 ml + 856 mls = 1781 mls = 1.781 L

Final [HA] = 0.7511 mols / 1.781 L = 0.4217 M

Final [A-] = 0.2910 mols / 1.781 L = 0.1634 M

Plugging values into the HH equation...

pH = 6.90 + log (0.1634 / 0.4217)

pH = 6.90 + log 0.3875

pH = 6.90 - 0.412

pH = 6.49