A mixture of $MgO$ and $Mg$ weighing $10$g is treated with excess of dil.$HCl$. Then $2.24$ lit of $H_2$ gas was liberated under STP conditions. The mass of $MgO$ present in the sample is

Under the standard conditions the volume of one mole of substance is $22.4$lit

Atomic mass of magnesium=$24$

Weight of $MgO$ =$2.24 x 24/ 22.4$=$2.4$g

Total weight of $MgO$ =$10$g

So, $10-2.4g= 7.6g$

Therefore the mass of $MgO$ is $7.6$g.

Hence the correct option is B.