How many protons are there in 2.5 l of liquid water?

The density of liquid water is 1 kg/L. 

Mass of water = (1 kg/L)(2.5 L)(1000 g/1 kg) = 2500 g water

The molar mass of water is 18 g/mol.

Moles of water = 2500 g / 18 g/mol = 138.889 mol water

According to Avogadro's number, in one mole of any substance, there are 6.022×10²³ atoms.

Atoms of water = 138.889 mol * 6.022×10²³ atoms/mol = 8.364×10²⁵ atoms of water

In every atom of water, there are 10 protons: 8 from one oxygen atom, and 1 each from the two hydrogen atoms.

Protons of water = 8.364×10²⁵ atoms * 10 protons/atom
Protons of water = 8.364×10²⁶ 

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