Everything Science Grade 11

6.7 CHAPTER 6. QUANTITATIVE ASPECTS OF CHEMICAL CHANGE

6.7 Molar concentrations of liquids ESBBM

A typical solution is made by dissolving some solid substance in a liquid.The amount of substance that

is dissolved in a given volume of liquid is known as the concentration of the liquid. Mathematically,

concentration (C) is defined as moles of solute (n) per unit volume (V) of solution.

C =

n

V

For this equation, the units for volume are dm^3. Therefore, the unit ofconcentration is mol.dm−^3.

When concentration is expressed in mol.dm−^3 it is known as themolarity (M) of the solution. Molarity

is the most common expression for concentration.

Tip

Do not confuse molar-
ity (M) with molar mass
(M). Look carefully at the
question in which the
M appears to determine
whether it is concen-
tration or molar mass.
Sometimes you will see
molar mass written as
Mm.

DEFINITION: Concentration

Concentration is a measure of the amount of solute that is dissolved in agiven volume

of liquid. It is measuredin mol.dm−^3. Another term that is used for concentration is

molarity (M)

Example 14: Concentration Calculations 1

QUESTION

If 3.5 g of sodium hydroxide (NaOH) is dissolvedin 2. 5 dm^3 of water, what is the concentration

of the solution in mol.dm−^3?

SOLUTION

Step 1 : Convert the mass of NaOH into moles

n =

m

M

=

3. 5

40

= 0. 0875 mol

Step 2 : Calculate the concentration of the solution.

C =

n

V

=

0. 0875

2. 5

= 0. 035 M

The concentration of thesolution is 0. 035 mol· dm−^3 or 0.035 M