${H}_{4} {P}_{2} {O}_{7}$
$2 {x}+4(+1)+7(-2)=0$
${x}=+5$
${H}_{4} {P}_{2} {O}_{5}$
$2 {x}+4(+1)+5(-2)=0$
${x}=+3$
${H}_{4} {P}_{2} {O}_{6}$
$2 {x}+4(+1)+6(-2)=0$
${x}=+4$
$(I)$ Benzylchloride
$(II)$ $p-$ Methoxy benzyl chloride
$(III)$ $p-$ Nitro benzyl chloride

${C_2}{H_5}I\xrightarrow{{Alco.\,\,KOH}}X\xrightarrow{{B{r_2}}}Y\xrightarrow{{KCN}}Z$