Question

Calculate the volume, in milliliters, of a 0.380 M KOH solution that should be added to 4.250 g of HEPES (MW = 238.306 g/mol, pKa = 7.56 ) to give a pH of 7.20.

Answer 1

Answer: 29.3 ml

Explanation: HEPES is a zwitterion. That is, it has both the acid and base components in the same molecule. However, we can write its formula as HA. Then the equation for the equilibrium is

MM: 238.306

HA + H₂O ⇌ H₃O⁺ + A⁻; pKₐ = 7.56

m/g: 6.00

1. Calculate the moles of HEPES

2. Calculate the concentration ratio.

We can use the Henderson-Hasselbalch equation.

The acid and its conjugate base are in the same solution, so the concentration ratio is the same as the mole ratio.

3. Calculate the moles of HA and A⁻

4. Calculate the moles of KOH

We are preparing the buffer by adding KOH to convert HA to A⁻. The equation is

HA + OH⁻ ⟶ A⁻ + H₂O

The molar ratio is 1 mol A⁻:1 mol OH⁻, so we must use 0.009 742 mol of KOH.

5. Calculate the volume of KOH