Question

If you have 120. mL of a 0.15 M MOPS buffer at pH 7.20 and you add 2.00 mL of 1.0 M HCl, what will be the new pH? (The pKa of MOPS is 7.20.)

Answer 1

Answer – We are given, pH = 7.20 , [MOPS] = 0.15 M , volume = 120 mL

[HCl] = 1.0 M , volume = 2.0 mL , pKa = 7.20

So, for the MOPS there is conjugate base/acid ratio is 1 and pH = pKa

Moles of MOPS = 0.15 M * 0.120 L = 0.018 moles

Moles of HCl = 1.0 M * 0.002 L = 0.002 moles

When we added the HCl there is moles of acid increase and moles of conjugate base decrease

Moles of acid = 0.018 +0.002 = 0.020 moles

Moles of conjugate base = 0.0180 -0.0020 = 0.016 moles

Total volume = 120+2 = 122 mL

[acid] = 0.020 mole / 0.122 L = 0.164 M

[conjugate base] = 0.016 moles / 0.122 L = 0.131 M

We know Henderson Hasselbalch equation –

pH = pKa + log [conjugate base] / [acid]

      = 7.20 + log 0.131 M / 0.164M

      = 7.10