State Avogadro's Law.

A cylinder contains 68 g of ammonia gas at s.t.p.

(i) What is the volume occupied by this gas?

(ii) How many moles and how many molecules of ammonia are present in the cylinder?

[N = 14, H = 1]

By Avogadro's law: Under the same conditions of temperature and pressure equal volumes of all gases contain the same number of molecules.

(i) Gram molecular mass of ammonia = N + 3(H) = 14 + 3 = 17 g.

17 g. occupies 22.4 lit. of vol.

∴ 68 g will occupy = $\dfrac{22.4}{17}$ x 68 = 89.6 lit.

Hence, volume occupied by this gas = 89.6 lit.

(ii) 17 g = 1 mole

∴ 68 g = $\dfrac{1}{17}$ x 68 = 4 moles.

1 mole = 6 × 10²³

∴ 4 moles = 4 x 6.023 × 10²³ molecules.

Hence, Moles = 4 and molecules = 4 x 6.023 × 10²³ molecules