A 0.55 g sample of H20 contains how many

molecules of water?

Answer in units of molec.

Num of molecules = num of moles * Avogadro's constant (6.02 * 10^23)

But num of moles = reacting mass / molar mass

Molar mass of H20 = 2*1 + 16 = 2+16 = 18g

Reacting mass of H20 = 0.55g

Therefore, num of moles of H20 = 0.55g/18g = 0.031 moles

Therefore, num of molecules of H20 = 0.031 * 6.02*10^23

= 1.87*10^22 molecules of H20