PHYSICAL CHEMISTRY IN BRIEF

CHAP. 1: BASIC TERMS [CONTENTS] 40

1.6.4 Amount-of-substance concentrationci.

Definition
ci=ni/V , (1.13)
whereV is the total volume of the mixture.
U Main unit: mol m−^3. In a pure component this quantity is identical with the amount-of-
substance density.

Note:The expression amount-of-substance concentration is usually abbreviated to amount

concentration or substance concentration. The same applies to the expression amount-

of-substance density. When there is no risk of ambiguity, the word concentration may

be used alone. In older literature, the term molarity may be found indicating the same

quantity (using the unit mol dm−^3 ).

Example

A mixture of substances with a volume of 5 dm^3 contains 56 g of nitrogen. Calculate its concen-

tration.

Solution

The amount of substance of nitrogen is

nN 2 =

56

28

= 2mol.

From equation (1.13) we obtain

cN 2 =

2

5

= 0. 4 mol dm−^3.

1.6.5 Molalitymi

Definition

mi=ni/msolvent (1.14)