3 questions im Stuck on in chemistry class?

Question

a 25.3g sample of an unknown compound contains 0.8g of oxygen. what is the percent by *** of oxygen in the compound?

magnesium combines with oxygen to form magnesium oxide. if 10.57g of magnesium reacts completely with 6.96g of oxygen, what is the percent by mass of oxygen in magnesium oxide?

when mercury oxide is heat,decomposess into mercury and oxygen. if 28.4g of mercury oxide decomposes,producing 2.0g oxygen,what is the percent by mass of mercury in mercury oxide?

Answer 1

% O = 0.8 x 100 / 25.3 = 3.16

Moles Mg = 10.57 g / 24.3050 g/mol =0.435

moles O = 6.96 g / 15.9994 g/mol = 0.435

the ratio is 1 : 1 and the formula is MgO

% O = 2.0 x 100 / 28.4 = 70.4

% Hg = 100 - 70.4 = 29.6