Chemistry, Third edition

140 9 · CALCULATING CONCENTRATIONS

Example 9.3

How much of a sodium chloride solution, of concentration

0.500 mol dm^3 , would you take and dilute in order to make

exactly 500 cm^3 of a solution of concentration 0.100 mol dm^3?

Answer

First, work out how many moles of sodium chloride you would end up with.

Since the amount of NaCl in moles volumethe molar concentration,

500 cm^3 of a solution of NaCl of concentration 0.100 mol dm^3 contains

500

0.1000.0500 mol

1000

What volume of the solution of concentration 0.500 mol dm^3 does this

correspond to?

volume

amount of NaCl in mol

molar concentration



0.0500

0.100 dm^3 or 100 cm^3

0.500

Comment

You would therefore take 100 cm^3 of solution of concentration 0.500 mol dm^3

and dilute it to 500 cm^3 in a volumetric flask.

Making up standard solutions

(i) What mass of anhydrous sodium carbonate would you have to weigh out to make 500 cm^3

of a standard solution of concentration 0.010 mol dm^3?

(ii)If, when making the solution in (i), you did not add enough water to bring the volume of

the solution completely up to the 500 cm^3 graduation mark on your volumetric flask, what

effect would that have on the concentration of the standard solution?

(iii)Calculate the concentration of a solution that results when 50 cm^3 of a sodium hydroxide

solution, of concentration 0.1 mol dm^3 , is pipetted into a 250 cm^3 volumetric flask and

made up to the graduation mark with deionized water.

(iv)You have some acid of concentration 16.0 mol dm^3. How much of the concentrated acid

would you dilute in order to make 1000 cm^3 of acid solution at a concentration of

2.00 mol dm^3?

(v)How much of a copper(II) sulfate solution, of concentration 2 mol dm^3 , would you need to

take and dilute to 200 cm^3 in order to end up with a solution of concentration

0.5 mol dm^3?

Exercise 9D

Volumetric analysis

The procedure involved in finding the concentration of a solution is called volu-

metric analysis. It involves reacting a solution of knownconcentration (a standard

solution) with one of unknownconcentration, in order to determine the equivalence

point.

9.3