Calculate the volume, in milliliters, of 0.170 M NaOH that must be added to 397 mL 0.0591 M 3‑( 𝑁 ‑Morpholino) propanesulfonic acid (MOPS) to give the solution a pH of 7.55.

Question

Calculate the volume, in milliliters, of 0.170 M NaOH that must be added to 397 mL 0.0591 M 3‑(𝑁‑Morpholino) propanesulfonic acid (MOPS) to give the solution a pH of 7.55. The p𝐾a of MOPS is 7.18.

Answer 1

Using the Henderson Hasselbalch equation

pH = pKa + log [A-]/[HA] where A- is the conjugate base of MOPS and MOPS is a weak acid

7.55 = 7.18 + log [A-]/[HA]

log [A-]/[HA] = 0.37

A / HA = 2.34

initial mols HA = 0.397 L x 0.0591 mol/L = 0.0235 mols

A / 0.0235 - A = 2.34

A = 0.0165 mols = mols NaOH

Volume of NaOH = 0.0165 mols x 1 L / 0.17 mols = 0.0970 L = 97.0 mls of NaOH need to be added