Calculate pH of 250 ml solution containing 325 mg aspirin (acetylsalicylic acid).

General information:

Aspirin ( molecular weight = 180.15 g/ mole ) dissociates in water to produce acetylsalicylate and hydronium ions. It has pKa value equal to 3.49 ( Ka = 3.27 × 10^-4 ) at 25 °C.

Calculation:

Firstly, molar concentration is calculated as the following:

weight in grams = 325 mg × 1 (g /1000 mg) = 0.325 g

number of moles = 0.325 g / (180.15 g/ mole) = 0.0018 mole

volume of solution in liters = 250 ml  × 1 (liter/ 1000 ml) = 0.25 liter

molar concentration = 0.0018 mole/ 0.25 liter = 0.0072 M

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

pH = 2.86.