Calculate pH of 1 liter solution containing 0.5 g benzoic acid.

General information:

Benzoic acid has a molecular weight of 122.12 g/ mole. It dissociates in water to produce benzoate and hydronium ions with pKa value equal to 4.20 (Ka = 6.30×10^-5) at 25 °C.

Calculation:
Firstly, molar concentration  of the acid is calculated as the following:

1 liter solution contains 0.5 g

number of moles = 0.5 g/ molecular weight

number of moles = 0.5 g/ (122.12 g/ mole)

number of moles = 0.0041 moles

molar concentration = number of moles/ volume of solution

molar concentration = 0.0041 moles/ 1 liter

molar concentration = 0.0041 M

Next, pH is calculated using online pH calculator or by using the equation used for calculating pH of solutions containing weak acids as the following:

Result: pH =3.32.