P Block Elements

When white phosphorus (P4) is heated with concentrated NaOH solution in an inert atmosphere of CO2, we are told it gives a salt ' X ' and a gas ' Y '.
Step 1: Recall the known reaction
White phosphorus reacts with hot concentrated NaOH (and no oxygen) to produce phosphine gas (PH3) and sodium hypophosphite (NaH2PO2) as the salt:
P4+3NaOH+3H2O→PH3+3NaH2PO2
Step 2: Identify ' X ' and ' Y '
Salt X=NaH2PO2 (sodium hypophosphite)
Gas Y=PH3 (phosphine)
Step 3: Find oxidation states of phosphorus in each
For X=NaH2PO2
Let oxidation state of P=x.
‌(+1)+2(+1)+x+2(−2)=0
‌1+2+x−4=0
‌x=+1
So in NaH2PO2,P has oxidation state +1.
For Y=PH3
Let oxidation state of P=x.
x+3(+1)=0x=−3
Thus, in PH3,P has oxidation state −3.