How many atoms are there in 5 litres of water

If we take 5 litres of water as having mass 5,000g :
Molar mass H2O = 18g/mol
5,000g = 5,000/18 = 277.78 mol H2O
1 mol H2O contains 6.022*10^23 molecules
277.78 mol contains 277.78*6.022*10^23 molecules
Each molecule of H2O contains 3 atoms
number of atoms = 3*277.78*6.022*10^23 = 5 *10^26 atoms.

At what temperature? You need the density of water to calculate the mass and density varies with temperature.

Assuming density is 1kg per litre
You have 5 kilos of water
1 mole of water is 18 grams
Calculate the number of moles in 5kg
Then multiply the number of moles by Avogadro's constant.
That gives you the number of molecules of water
Each molecule is 3 atoms
Multiply the number of molecules by 3 to get the number of atoms

lets go for 4 °C, 10^5 Pa ultrapure water. (so 1 gram = 1 ml)


5000/18.02 * 6,02214E23 *3 = a rough estimate of the amount of atoms.

5000 grams of water
molar mass of water = 18,02
Avogadro constant = 6,02214E23
3 atoms per molecule