Chemistry, Third edition

9 · CALCULATING CONCENTRATIONS

Concentration expressed as mole fraction

Sometimes the physical properties of solutions, such as vapour pressure(a measure of the

number of molecules escaping from the surface of the liquid, see page 165), depend upon

the relative numbers of particles of the components of the solution. In these cases, the

composition of a solution can be described in terms of the mole fractionof each of the

components. The mole fraction of any component is calculated by dividing the number

of moles of that component by the total number of moles present in the mixture.

For example, in a mixture of A and B, the mole fraction of A (symbol XA), is

defined as follows:

nA

XA

nAnB

wherenAis number of moles of A present and nBis number of moles of B present.

Similarly, for B,

nB

XB

nAnB

(Note that XA+XB1.)

Also note that:

1.A mole fraction has no units, it is a ratio.

2.A mole fraction has a value between 0 and 1.

150

Calculating mole fractions

(i)A mixture consists of 1 mol of ethanol (C 2 H 5 OH), 3 mol of methanol (CH 3 OH) and 6 mol of

water. What is the mole fraction of methanol present?

(ii)A mixture consists of 37.25 g of potassium chloride dissolved in 1000 g of water. What is the

mole fraction of potassium chloride present?

Exercise 9M

Mole fractions in a four-component mixture

(i) In a four-component mixture of P, Q, R and S, write an expression for the mole fraction of

R,XR.

(ii)Show that XP+XQ+XR+XS1.

(iii)What is the composition of the mixture if XP1?

Exercise 9L

pH scale

The pH scale is a way of expressing the [H 3 O], or [H(aq)], as a smallnumber.

Note that the square brackets, [ ], stand for ‘the concentration of ’ (in mol dm^3 ).

The pH of a solution can be calculated using the equation

pHlog [H(aq)]

Alternatively, the hydrogen ion concentration can be calculated using the equation

[H(aq)] 10 pH

9.5