Question

The active ingredient in aspirin is acetylsalicylic acid. A 2.79 g sample of acetylsalicylic acid required 30.15 mL of 0.5137 M NaOH for complete reaction. Addition of 17.41 mL of 0.4448 M HCl to the flask containing the aspirin and the sodium hydroxide produced a mixture with pH = 3.48. Find the molar mass in g/mol) of acetylsalicylic acid and its K, value. Acetylsalicylic acid is a monoprotic acid. molar mass 49 g/mol Ka

Answer 1

Molar mass = 180 g/mol

Ka = 3.31x10⁻⁴

Step-by-step explanation:

Because the acetylsalicylic is a monoprotic acid, it only leaves 1 mol of H⁺ in the reaction, so, it's necessary 1 mol of NaOH for each 1 mol of acetylsalicylic acid. The number of moles of NaOH is the volume, in L, multiplied by the concentration:

n = 0.03015*0.5137 = 0.01549

Which is also the number of moles of the acid. The molar mass is the mass, in g, divided by the number of moles:

M = 2.79/0.01549

M = 180 g/mol

After reacting with the base, the conjugate base of the acid will be formed, and the equilibrium between the acid form (HA) and the conjugate base (A⁻) is a buffer. It formed 0.01549 moles of A⁻, which will react with the moles of H⁺ from HCl to form HA.

nH⁺ = nHCl = 0.01741*0.448 = 0.0078 mol

A⁻ + H⁺ ⇄ HA

0.01549 0.0078 0 Initial

-0.0078 -0.0078 +0.0078 Reacts

0.0078 0 0.0078 Equilibrium

For a buffer, the pH can be calculated by the Handerson-Halsebach equation:

pH = pKa + log[A⁻]/[HA]

Because the volume is the same, we can use the number of moles instead of the concentration:

3.48 = pKa + log(0.0078/0.0078)

pKa = 3.48

pKa = -logKa

Ka =
`10^(-pKa)`

Ka =
`10^(-3.48)`

Ka = 3.31x10⁻⁴