I₁, I₂, I₃ of an element are 176, 346 and 1850K. calls respectively. The molecular formula of its phosphate is

From the given Ionization values, we can observe sudden jump from I₂ to I₃. 

So, The valence of element(M)=2

${\mathrm{M}}^{+2} {\mathrm{PO}}_4^{-3}$

Hence the formula is ${\mathrm{M}}_3{\left({\mathrm{PO}}_4\right)}_2$