The number of grams of NaOH that completely neutralizes 4.9 gm of phosphoric acid is

H₃PO₄ + 3NaOH$\underset{}{\overset{}{\to }}$  Na₃PO₄  + 3H₂O

               98 gm H₃ PO₄ $\underset{}{\overset{}{\to }}$ 3 x 40 gm NaOH

               4.9 gm H₃PO₄$\underset{}{\overset{}{\to }}$ ?

                = $\frac{3\times 40}{\cancel{98}}\times \cancel{4.9}=6gm$