To completely neutralize a solution containing 0.1 mol of orthophosphoric acid, sodium alkali with a mass of

H3PO4 + 3NaOH = Na3PO4 + 3H2O

According to the stoichiometry of the reaction, the amount of alkali substance should be three times more than the amount of acid substance:

n (NaOH) = 3n (H3PO4) = 3 * 0.1 = 0.3 mol;

Find the mass of sodium hydroxide:

m (NaOH) = n (NaOH) * M (NaOH) = 0.3 * 40 = 12 g.

Answer: you need 12 g of caustic soda.