Answer:

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.
![\begin{array}{rcl}\text{pH} &= &\text{pK}_{\text{a}} + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\\7.36 & = & 7.56 + \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\\-0.20 & = & \log\dfrac{[\text{A}^{-}]}{\text{[HA]}}\\\\\dfrac{[\text{A}^{-}]}{\text{[HA]}} & = & 0.631\\\\\end{array}](https://tex.z-dn.net/?f=%5Cbegin%7Barray%7D%7Brcl%7D%5Ctext%7BpH%7D%20%26%3D%20%26%5Ctext%7BpK%7D_%7B%5Ctext%7Ba%7D%7D%20%2B%20%5Clog%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%5C%5C%5C%5C7.36%20%26%20%3D%20%26%207.56%20%2B%20%5Clog%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%5C%5C%5C%5C-0.20%20%26%20%3D%20%26%20%5Clog%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%5C%5C%5C%5C%5Cdfrac%7B%5B%5Ctext%7BA%7D%5E%7B-%7D%5D%7D%7B%5Ctext%7B%5BHA%5D%7D%7D%20%26%20%3D%20%26%200.631%5C%5C%5C%5C%5Cend%7Barray%7D)
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
