Question

number of atoms present in 1 gram of H2

Answer 1

A mole is a convenient counting unit whenever one is dealing with numbers of atoms or molecules. It is equal to Avogadro's number (NA), namely 6.022x1023. If we have one mole of water, then we know that it will have a mass of 2 grams (for 2 moles of H atoms) + 16 grams (for one mole O atom) = 18grams