What is the molar mass of the weak acid?

Question

An unknown, solid, monoprotic weak acid (253.2 mg) was dissolved in 10 mL of water and titrated using 0.1355 M NaOH. If it took 11.18 mL of the NaOH solution to reach the end point, what is the molar mass of the weak acid.

Answer 1

Unknown acid = H⁺

H⁺ + OH^- ==> H2O

moles OH^- used = 11.18 mls x 1 L / 1000 mls x 0.1355 mol/L = 0.00151 mols OH^- used

moles H⁺ present = 0.00151 mols OH^- x 1 mol H⁺ / mol OH^- = 0.00151 mols H⁺ present

molar mass of unknown = 253.2 mg x 1 g / 1000 mg / 0.00151 mols = 167.7 g/mol = molar mass