What is the mass of 22.4 liters of H2 gas at STP?

The number of hydrogen atoms present in 5.4 g of urea is: (Given: Molar mass of urea = 60 g mol-1, NA = 6.022 × 1023 particles mol-1)

How many molecules of H2O2 are present in 68 g of H2O2? (Molar mass of H2O2 = 34 g/mol)

If X is a haloalkane with a single chlorine atom per molecule and the mass percentage of Cl is 55%, what is the number of Cl atoms present in 0.1 g of the haloalkane? (Atomic mass of Cl = 35.5 g/mol, NA = 6.022 × 1023 mol-1)