Calculate pH of 0.1% w/v methylparaben sodium solution.

General information:
Methylparaben sodium (molecular weight = 174.13 g/ mole) is a salt of methylparaben. Methylparaben is a weak acid and has a pKa value of 8.4 at 25 ˚C. When methylparaben sodium dissolves in water it acts as a weak base (pKb = 14 - 8.4 = 5.6). Behavior of salts of weak acids in solution is discussed in a previous post.

Calculation:
The first step is to find the molar concentration of 0.1 % w/v as the following:

0.1 % w/v = 0.1 g/ 100 ml

 Next, pH is calculated using online pH calculator or by applying the equation used for pH calculation of a weakly basic compound as the following (pKb = 5.6, Kb = 2.51×10^-6):

The pH is in the alkaline region > 7 because the salt acts as a weak base.

Related post:

Calculate pH of 0.1% w/v methylparaben solution.

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