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The equivalent mass of H3PO4 and Na2HPO4 in the reaction are respectively:
2NaOH+H3PO4Na2HPO4+2H2O
A. 49,142
B. 49,71
C. 98,71
D. 98,142

Answer
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Hint : We know that equivalent mass is determined by the formula that Equivalent mass = Molar mass / n- factor. To calculate n factor of a salt in a reaction we calculate the total moles of cationic or anionic charge replace in one mole of the salt or we can say that one mole of reactant is taken and this would will help in finding in the number of moles of elements whose oxidation state is changing.

Step-By-Step answer:
Molar mass of H3PO4 =3× atomic mass of hydrogen + atomic mass of phosphorus +4× atomic mass of oxygen =3×1+31+4×16=98 gram per mole. Since here two hydrogen ions are replaced in the reaction so n factor is two. Hence equivalent weight of H3PO4 = Molar mass of H3PO4/ n factor =982=49 gram per equivalent.
Molar mass of Na2HPO4 = 2×atomic mass of sodium + atomic mass of hydrogen + atomic mass of phosphorus +4× atomic mass of oxygen =2×23+1+31+4×16=142 gram per mole.
Here the n factor of salt Na2HPO4 is two because the total moles of cationic or anionic charge replaced in one mole of the salt is two. Hence equivalent weight Na2HPO4 = Molar mass of Na2HPO4 / n- factor =1422=71 gram per equivalent.

Hence option B is correct, that is 49,71.

Note : We have calculated equivalent weight of H3PO4 and Na2HPO4 by calculating n- factor of them. First of all we have to calculate the molar mass of each compound. Thus we got equivalent mass. Na2HPO4 reacts with two moles of monobasic acid so n factor is two here while two hydrogen ion is replaced in order to form Na2HPO4 so n-factor of H3PO4 is also two. We know that the equivalent weight of H3PO4 and Na2HPO4 will be the ratio of their corresponding molar mass to n factor.
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