Calculate the number of molecules in 109.5 g of HCl. [H = 1, Cl = 35.5]

gm mol. wt. of HCl = 1 + 35.5 = 36.5 g

At s.t.p.,

36.5 g of HCl = 6.023 x 10²³ number of molecules    [Avogadro's law]

Therefore, 109.5 g of HCl

$= \dfrac{6.023 \times 10^{23}}{36.5} \times 109.5 \\[0.5em] = \dfrac{109.5}{36.5} \times 6.023 \times 10^{23} \\[0.5em] = 3 \times 6.023 \times 10^{23} \\[0.5em]$

Hence, number of molecules in 109.5 g of HCl is 3 x 6.023 x 10²³