First determine the number of moles of O2 obtained using the ideal gas law. To use the gas constant 0.0821 L-atm/mole-degree, the pressure and volume must be changed to atmospheres and liters and the temperature changed to degrees K.
To find the number of moles use n = PV / RT = 735/760 x 0.254 / 0.082 x 305.15 = 0.00981 moles O2.
The balanced equation is 2 HgO ====> 2Hg + O2 so a conversion factor to get grams HgO from moles O2 is 2 x 216.59 g HgO / 1 mole O2 = 433.18. The grams of HgO then is 433.18 x 0.00981 = 4.249 g HgO required to produce 0.00981 moles of O2 by decomposition.
The percent of HgO in the mixture is then 4.249 / 4.85 x 100 = 87.6%.
I left out some units for simplification.