What should be the equivalent weight of phosphorous acid,if $P=31, O=16, H=1$?

The equivalent weight of $K_2Cr_2O_7$ in acidic medium is:

Assertion : The normality of $0.3 \ M$ aqueous solution of $H_3PO_3$ is equal to $0.6 \ N$.
Reason : Equivalent weight of $H_3PO_3 = \frac{\text{Molecular weight of } H_3PO_3}{3}$

Calculate the molecular mass of the following:
$(i)$ $H_{2}O$
$(ii)$ $CO_{2}$
$(iii)$ $CH_{4}$

The relative abundances of two isotopes of rubidium with atomic masses $85$ and $87$ are $75\%$ and $25\%$ respectively. What is the average atomic mass of rubidium (in $.5$)?

$H_2SeO_4 + NaOH \to NaHSeO_4$; $E.W.$ of $H_2SeO_4 = E_1$
$H_2SeO_4 + 2NaOH \to Na_2SeO_4$; $E.W.$ of $H_2SeO_4 = E_2$
What is the relation between $E_1$ and $E_2$?