#"Mass"##=##(2xx31.00+5xx15.999)*g*mol^-1=??*g*mol^-1#. All I have done here is to take the atomic masses and mulitplied them appropriately. From where did I get the atomic masses?
Note that #P_2O_5# is an empirical formula. The actual beast is #P_4O_10#, which has a molecular mass of #283.89*g*mol^-1#.