Question

Find the mass of 1 billion molecules of water.

Answer 1

You have to first convert the number of water molecules to moles (1 mol is 6.022 x 10^23). the molecular weight of water is 18 g/mol, which is the weight of water when there is 1 mol of water. Hence, divide 10^18 by the avogadro's number (6.022 x 10^23) to find out how many mol of water you have. Take the number of mol of water and multiply by the molecular weight of water. Voila!

Answer 2

Use avogadro's constant to first convert to mole, then multiply by molar mass of water.