On dissolving 1 mole of each the following acids, in 1 litre water, the acid which does not give a solution of strength 1N

1mole dissolved in 1l given 1N sol

This is possible only when valency of the acid=1

HCl, HNO_3,HClO_4......  monobasic $\therefore 1\mathrm{M}=1\mathrm{N}$

But H₃PO₄ is tribasic 

$\therefore 1\mathrm{M}\ne 1\mathrm{N}$

Thus 1 mole of H₃PO₄  dissolved in one lit gives 1M sol. but not 1N sol