Chemistry, Third edition

136 9 · CALCULATING CONCENTRATIONS

In general for solutions,

amount of substance in mol volumemolar concentration

When using this equation, volume is expressed in dm^3 and concentration is in

mol dm^3.

Rearranged forms of this equation may be needed:

amount in mol

volume

molar concentration

or

molar concentration 

amount in mol

volume

Example 9.1

What is the concentration of a solution of sodium hydroxide

(NaOH) that contains 0.1 mol of NaOH dissolved in 250 cm^3 of

solution?

Answer

amount of NaOH in mol volumemolar concentration

Substituting, and remembering that 1 dm^3 1000 cm^3 , so we must divide

250 cm^3 by 1000 to convert it to dm^3 ,

0.1

250

molar concentration

1000

Therefore the concentration of the NaOH solution is

1000 0.1

0.4 mol dm^3

250

Example 9.2

How many grams of sodium chloride are there in 500 cm^3 of a

solution which has a concentration of 0.500 mol dm^3?

Answer

amount of NaCl in molvolumemolar colcentration



500

0.500

1000

0.250 mol

To convert moles into grams:

amount of NaCl in mol

mass NaCl

molar mass NaCl

0.250

mass NaCl

58.5

which gives

mass of NaCl 0.25058.514.6 g

Therefore, 14.6 g of NaCl are dissolved in 500 cm^3 of solution.