Answer : To determine the mass of oxygen in the compound produced in the virtual lab, it should be weighed and subtracted from the total weight of the compound. The mass pf each element can be used to determine the empirical formula of the compound by finding out the molar ratios of the individual elements present in the compound. Molar ratios can be obtained by dividing elements by atomic masses of individual elements.
One has to spot the smallest moles of the elements present in the compound and then it has to be divided by rest of the elements in the compound to find the empirical formula of that compound.