How many molecules of water are present in 5g sample?

In order to measure the amount of water molecules, we need to use following equation:
n = m / M where n is the amount of water molecules in molesm is the mass in grammesM is the molecular mass of water
Water consists of two hydrogen atoms connected to one oxygen atom, therefore molecular mass of water (M) is equal to sum of molecular masses of oxygen and two hydrogen atoms which is 2x1.01 +15.99. Therefore molecular mass of water (M) is equal to 18,01.
This allows to calculate number of moles of water in aforementioned sample:
n = 5 g / 18.01 gmol^-1n = 0.278 mol
In order to calculate amount of water molecules we need to multiply result in moles (0.278) by Avogadro's constant which represents amount of molecules in one mole of a certain substance:
0.278 x 6.02 x10²³mol^-1= 1.67 x 10²³
So in 5g sample of water there is 1.67 x 10²³ water molecules.