Question

Calculate the pH of a solution formed by dissolving 0.040 moles of solid sodium lactate, NaC3H5O3, in 50.0 mL of 1.0 M HC3H5O3. Assume that any volume change is negligible.

Answer 1

The pH of the solution is 3,76

Why?

To solve this problem we have to apply the Henderson-Hasselbach equation, which is used whenever we need to calculate the pH of a solution of an acid HA (Lactic Acid) and its conjugate base A⁻ (Sodium Lactate)

We can use either moles or concentrations for this equation. In this case, we are going to use the moles, and we are going to take the pKa of lactic acid as 3,86:

$pH=pKa+log(\frac{nA^- }{nHA} )=3,86+log(\frac{(0,04moles)}{(0,05mL)*(1.0M)} )\\\\pH=3,76$

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