How many atoms of hydrogen are present in 2.92 g of water?

The molecular weight of water is 18.01528 g/mol.

So in 2.92 grams there are 2.92 / 18.01528 = 0.1621 mol of particles.

1 mol contains 6,02214 * 10^ 23 particles by definition.

So the nr of H2O molecules is 0.1621 * 6,02214 * 10^23 = 0,9761 * 10^23.

Every molecule has 2 H atoms, so you have to double that.

2 * 0,9761 * 10^23 = 1.952 * 10^23.