P Block Elements

When white phosphorus $\left(\mathrm{P}_4\right)$ is heated with concentrated $\mathrm{NaOH}$ solution in an inert atmosphere of $\mathrm{CO}_2$, we are told it gives a salt ' $X$ ' and a gas ' $Y$ '.Step 1: Recall the known reactionWhite phosphorus reacts with hot concentrated $\mathrm{NaOH}$ (and no oxygen) to produce phosphine gas $\left(\mathrm{PH}_3\right)$ and sodium hypophosphite $\left(\mathrm{NaH}_2\mathrm{PO}_2\right)$ as the salt:$\mathrm{P}_4+3\mathrm{NaOH}+3\mathrm{H}_2\mathrm{O}\rightarrow\mathrm{PH}_3+3\mathrm{NaH}_2\mathrm{PO}_2$Step 2: Identify ' X ' and ' Y 'Salt $X=\mathrm{NaH}_2\mathrm{PO}_2$ (sodium hypophosphite)Gas $Y=\mathrm{PH}_3$ (phosphine)Step 3: Find oxidation states of phosphorus in eachFor $X=\mathrm{NaH}_2\mathrm{PO}_2$Let oxidation state of $P = x$.$\;(+1)+2(+1)+x+2(-2)=0$$\;1+2+x-4=0$$\;x=+1$So in $\mathrm{NaH}_2\mathrm{PO}_2,\;P$ has oxidation state $+1$.For $Y=\mathrm{PH}_3$Let oxidation state of $P = x$.$x+3(+1)=0$$x=-3$Thus, in $\mathrm{PH}_3,\;P$ has oxidation state $-3$.