Which of the following properties of an element is not variable?

In terms of energy,$1 \ amu$ is equal to:

One liter of hard water contains $12.00 \, mg$ of $Mg^{2+}$. How many milliequivalents of washing soda are required to remove its hardness?

Among $10^{-9} \ g$ (each) of the following elements,which one will have the highest number of atoms?
Element: $Pb, Po, Pr$ and $Pt$

$2 \ g$ of metal carbonate is neutralized completely by $100 \ mL$ of $0.1 \ N$ $HCl$. The equivalent weight of metal carbonate is:

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