What is total number of molecules contained in 0.50 mole of o2 at STP

1 moles of O2 at STP has 6.02 X 10 ^ 23 molecules

.5 mole has 3.01 X 10 ^ 23 molecules

What is the total number of molecules in 1.0 mole of Cl2 (g) ?

by definition 1 mole of gas has Avogadros number of molecules = 6.02 x 10 ^ 23

What is the percent by mass of aluminum in Al2O3?

Al in 1 mole of Al2O3 = 27*2 g

1 mole of Al2O3 weighs 102 g

% Al = 54 / 102 = 52.92 %

What is the empirical formula of a compound that contains 92.3% carbon and 7.7% hydrogen by mass?

In 100 g of compound we have 92.3 g of C = 92.3 / 12 = 7.69

moles of H = 7.7

molar ratio of C : H = 7.96 : 7.7 = 1:1

empirical formula is CH