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