The mass of oxygen present in 0.5 mole of ${\mathrm{P}}_4{\mathrm{O}}_{10}$ is (Atomic of P is 31 and  O is 16)

One molecule of  P₄​O₁₀ contains 10 oxygen atoms 

So, 

Mass of oxygen present in 1 mole of P₄​O₁₀​

16×10=160gm

For 0.5 moles of P₄​O₁₀​=$\frac{160}{2}=80\mathrm{g}$