The $O-H$ bond length in $H_2O$ in the gas phase is: (in $pm$)

Which of the following statements is $INCORRECT$?

Find the number of $CORRECT$ order against mentioned property :-
$(i)$ $D_2O = H_2O$ (number of electrons)
$(ii)$ $D_2O > H_2O$ $(|\Delta H_f|)$
$(iii)$ $D_2O > H_2O$ $(|\Delta H_{vaporization}|)$
$(iv)$ $D_2O > H_2O$ (density)
$(v)$ $D_2O > H_2O$ (Molecular mass)

Which of the following is/are considered hard water?

Oxygen was discovered by

Discuss the principle and method of softening of hard water by synthetic ion-exchange.