Question

In theory, what approximate volume of 1 M HCl would need to be added to 0.01 moles of Tris base to produce a buffer at pH 7.5?

Answer 1

pKa of Tris base = 8.08

Using hessley henderbach equation

pH = pKa + log([Tris]/[Tris.HCl])

7.5 = 8.08 +  log([Tris]/[Tris.HCl])

log([Tris]/[Tris.HCl]) = -0.58

Initially, 0.01 moles of Trise base is present, the reaction will be

Tris + HCl ---------------- Tris.HCl

Moles of Tris left after reaction = 0.01 - x(assuming x moles of HCl)

moles of Tris.HCl formed = 0.01-x

(0.01-x)/x = 10^(-0.58) = 0.263

Solving the equation we get

x = 0.00791 moles = Number of moles of HCl

Volume of HCl(in L) * Molarity of HCl = Number of moles

Volume of HCl(in L) = 0.00791 L = 7.91 mL