Chemistry, Third edition

OTHER UNITS OF CONCENTRATION 149

Concentration as percentage composition

Many solutions in industry are expressed in one of two ways:

1.Percent by mass (or ‘weight’), often written as percentage

(w/w):

percentage composition by mass



mass of solute

 100

mass of solution

2.Percent by volume, often written as percentage (v/v):

percentage composition by volume



volume of liquid solute

 100

volume of solution

Aqueous concentrations in ppm

and ppb

(i) Drinking water is considered acceptable if it

contains less than 500 ppm of chloride ions

(Cl). What is this concentration in

mol dm^3?

(ii)A standard solution contains 1000 ppm of

chloride ions. If a 50 cm^3 sample of this

standard solution were diluted to 250 cm^3 ,

what would be the concentration of the new

solution?

(iii)An aqueous solution of silver ions (Ag) is of

concentration 0.0005 ppm. Express this

concentration in ppb.

BOX 9.4 Exercise 9J

Measuring the alcohol content in spirits

An old-fashioned way of measuring the alcohol (the chemical name is

ethanol) content in spirits is to quote the concentration of alcohol in

terms of degrees proof. The measurement was made by pouring the

spirit over gunpowder. If the gunpowder would not burn, the spirit was

‘under-proof’ – it contained too much water. The spirit that had a

concentration of ethanol that would justallow gunpowder to burn was

termed 100 proof spirit, written 100. A sample that is 100contains

50% ethanol (v/v), so 70whisky contains about 35% ethanol.

Percentage compositions

(i) Glucose sugar has the formula C 6 H 12 O 6. If

you dissolved 15.5 g of glucose in 100 g of

water, what would be the concentration of

glucose, expressed as a percentage

composition by mass?

(ii)A concentrated ammonia solution is labelled

as 28% (w/w). How many grams of ammonia

are in 150 g of the solution?

(iii)A wine bottle has 12% (v/v) written on the

label, this describes the ethanol content in

the wine. What volume of ethanol is in a 1.5 L

bottle of wine?

(iv)Strong vodka is 85proof. What volume of

ethanol does a 1 L bottle contain?

Exercise 9K

Label from a wine bottle clearly showing the alcohol content. Wines

generally contain between 8% and 14% ethanol.