2.0 THE MOLE: Empirical and Molecular formula

2.13 Atomicity and molar gas volume

In the combined gas equation,

PV/T = k (Constant)

the constant, k, applies to all gases. So, the same volume of different gases at the same temperature exert the same pressure (1 atm at standard conditions). Right?

That means, there is something basic and common to the same volume of different gases at the same conditions. But it cannot be mass because this (RMM) varies across gases. Neither can it be speed, because different gases diffuse at different rates. Nor can it be density. Why? Answer. Density also varies between gases; it is a reason why different gases diffuse at different rates. It is likely to be number of molecules. Avogadro confirmed this.

Avogadro^,s law: Equal volumes of gases at the same temperature and pressure contain the same number of molecules.

The number of molecules contained in 22.4 dm³ at s.t.p (or 24.0 dm³ at r.t.p) is called Avogadro^,s number (6.02 x 10²³) or 1 mole of molecules.

Questions 2.13

1. Determine (a) the mass, (b) number of moles of molecules and (c) number of moles of atoms contained in 1.12 dm³ of oxygen gas at s.t.p. (O = 16, molar gas volume at s.t.p = 22.40 dm³)
2. Calculate the mass of carbon (IV) oxide contained in 6 dm³ at r.t.p. (C = 12; O = 16; molar gas volume at r.t.p is 24.0 dm³)
3. What volume of oxygen gas at s.t.p. contains the same number of moles as there are carbon (IV) oxide molecules in 500 ml at r.t.p? (C = 12; O = 16; molar gas volume at s.t.p = 22.4 dm³; molar gas volume at r.t.p = 24.0 dm³).
4. Atomicity is the number of atoms in one molecule of a gas. A certain gas, X, has a relative atomic mass (RAM) of 16. If 2.4g of X occupies 1.2 dm³ at r.t.p, determine the atomicity of the gas (molar gas volume at r.t.p. = 24.0 dm³).

Answers to Questions 2.13