Question

If you have 120. mL of a 0.10 M TES buffer at pH 7.55 and you add 2.00 mL of 1.0 M HCl, what will be the new pH? (The pKa of TES is 7.55.)

Answer 1

Solution.

The formula of TES is:

The nitrogen of this compound can be protonated, TES is a weak base:

The pH can be found using Henderson-Hasselbach Equation:

The initial concentration is [TES]₀ = [TES] + [TES(H⁺)];

The total volume of a mixture is 0.120 + 0.002 = 0.122 L.

The amount of the protonated form is equal to the amount of HCl added:

n(TES(H⁺)) = n(HCl) = c(HCl)V(HCl) = 1.0*0.002 = 0.002 mol.

n₀(TES) = 0.1*0.120 = 0.012 mol.

[TES]₀ = 0.012/0.122 = 0.098 M;

[TES(H⁺)] = 0.002/0.122= 0.016 M;

[TES] = 0.098-0.016 = 0.082 M;

Answer: 8.26.