For a titration, 4.75 g of an unknown acid, HX, was put in 100.0 mL of water. It required 38.57 mL of 0.9876 M NaOH to complete the reaction. What is the molar mass of the acid?

Liters of base X molarity of base = moles of base = moles of acid. Moles of acid = grams of acid /mole wt of acid. There you go!