Question: 1 litre of water contains how many molecules?
Answer: One mole of water has a mass of 18 grams and a volume of 18 mL (as the density of water is 1 g per cm3). It contains Avogadro's number of molecules (6x 10^23). Thus 1 litre of water contains 1 / 18 x 1000 x 6 x 1^23 molecules (assume you can do the maths…)