Question

Consider the following oxyanions:
$PO_{4}^{3-}$,${{P}_{2}}O_{6}^{2-}$,$SO_{4}^{2-}$,$MnO_{4}^{-}$,$CrO_{4}^{-}$,${{S}_{2}}O_{5}^{2-}$,${{S}_{2}}O_{7}^{2-}$ and find the value of $R+Q-P$; where
P = Number of oxy anions having three equivalent $X-O$ bonds per central atom
Q = Number of oxy anions having two equivalent $X-O$ bonds per central atom
R = Number of oxy anions having four equivalent $X-O$ bonds per central atom

Answer 1

Hint: Oxy anions are polyatomic ions that are attached to one or more than one oxygen atom For the oxyanions, the charge is the same only the number of oxygen atoms varies. To approach this problem, first, we will have to draw the structure of oxyanions. From the structure, we can find P.Q, and R values respectively.

Complete Step by Step Answer:
Oxyanion is nothing but an anion containing oxygen. The naming of these oxyanions depends on the number of oxygen atoms attached to the central atom.

When there is greater number of oxygen present, the suffix ‘ate’ is added at the end of oxyanions such as nitrite ($NO_{3}^{-}$ ), and the suffix ‘item’ is added at the end of oxyanions such as nitrite ($NO_{2}^{-}$ ) in the presence of less number of oxygen.

The structure of different oxyanions is drawn one by one so that we can determine the P, Q, and R values.
Structures of oxyanions:


Here three oxy anions have three equivalent $X-O$ bonds where x denotes the central atom. In ${{P}_{2}}O_{6}^{2-},{{S}_{2}}O_{5}^{2-},{{S}_{2}}O_{7}^{2-}$, there are three $P-O$bonds, three $S-O$ and three $S-O$ bonds respectively. Thus these three oxy anions are referred to as P. ${{S}_{2}}O_{5}^{2-},{{S}_{2}}O_{7}^{2-}$ is referred to as Q as they have two equivalent $S-O$ bonds. $\text{PO}_{\text{4}}^{\text{3-}}\text{,SO}_{\text{4}}^{\text{2-}}\text{,MnO}_{\text{4}}^{\text{-}}\text{andCrO}_{\text{4}}^{\text{-}}$ are denoted as R as they have four equivalent $X-O$ bonds.
Here the number of oxyanions are $P=3,Q=2,R=4$
Therefore $R+Q-P=4+2-3=3$
Thus, $3$ is the correct answer.

Note: Carbonate ($CO_{3}^{2-}$), Nitrate ($NO_{3}^{-}$), Phosphate ( $PO_{4}^{3-}$), silicate( $SiO_{4}^{2-}$), Sulphate ($SO_{4}^{2-}$) etc are the main oxy anions in our human organisms. They are also found in the soil environment. Some oxyanions like Arsenate ($AsO_{4}^{3-}$), Borate ($BO_{3}^{3-}$), Selenate ($SeO_{4}^{3-}$ ), etc. are very much toxic and cause environmental pollution.