You dissolve 0.213 g of acid in 25.0 mL of water. To react the acidic solution to the endpoint you add 23.80 mL of 0.1024 M NaOH. Determine the molar mass of the acid.
moles NaOH = M x L = ?
We assume, I suppose, that this is a monoprotic acid; then moles acid = moles NaOH.
moles acid = grams/molar mass. Solve for molar mass.