What is the pH of a solution obtained by dissolving two extra-strength aspirin tablets, containing 470 mg of acetylsalicylic acid each, in 240 mL of water?

2.58

Explanation:

Each tablet has a mass of 470 mg. The mass of 2 tablets is 2 × 470 mg = 940 mg = 0.940 g

The molarity of acetylsalicylic acid (aa) is:

M = mass of aa / molar mass of aa × liters of solution

M = 0.940 g / 180.16 g/mol × 0.240 L

M = 0.0217 M

Acetylsalicylic acid is a weak acid, so we can find [H⁺] using the following expression.

[H⁺] = √(Ca × Ka)

where,

Ca: concentration of the acid

Ka: acid dissociation constant

[H⁺] = √(0.0217 × 3.24 × 10⁻⁴)

[H⁺] = 2.65 × 10⁻³ M

The pH is:

pH = -log [H⁺] = -log 2.65 × 10⁻³ = 2.58