Question: how many molecules are present in 1 kg of hydrogen gas?
Answer: 1 kg is equivalent to 1000g. 1 mol of anything is an Avogadro's constant, or 6.022x10^23 anythings. 1 mol is also equivalent to the molar mass of a compound or element. Therefore we can say that 1 molar mass of either H2 or O2 is equivalent to 6.022x10^23 molecules.