You carry out the titration of a weak diprotic solid acid. You take 178.0 mg of the solid acid and dissolve it in about 50 mL of distilled water. You titrate with 0.095 M NaOH and arrive at an endpoint at 25.20 ml. You know that at the final pH both protons have been titrated. What is the formula weight (molar mass) of the acid?

Explanation:

Equation of the reaction:

H2X(aq) + 2NaOH(aq) --> Na2X + 2H2O

Mass of acid = 0.178 g

Number of moles = molar concentration * volume

= 0.095 * 25.20 mL⋅1 L/1000 mL

= 0.00239 mol

From the equation of reaction,

2 moles of NaOH neutralises in 1 mole of H2X.

Therefore,

0.00239 mole of NaOH, number of koles of H2X = 2 * 0.00239 mol

= 0.0012 mole.

Molar mass = mass/number of moles

= 0.178/0.0012

= 148.7 g/mol.