Chapter 1: Moles and equations
We often need to calculate the mass of a substance present in a solution of known concentration and volume. To do this we: ■
QUESTION 16 a Calculate the concentration, in mol dm–3, of the following solutions: (Ar values: C = 12.0, H = 1.0, Na = 23.0, O = 16.0)
rearrange the concentration equation to: number of moles (mol) = concentration (mol dm–3) × volume (dm3)
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i
multiply the moles of solute by its molar mass
a solution of sodium hydroxide, NaOH, containing 2.0 g of sodium hydroxide in 50 cm3 of solution
ii a solution of ethanoic acid, CH3CO2H, containing 12.0 g of ethanoic acid in 250 cm3 of solution.
mass of solute (g) = number of moles (mol) × molar mass (g mol–1)
b Calculate the number of moles of solute dissolved in each of the following:
WORKED EXAMPLE
i
19 Calculate the mass of anhydrous copper(II) sulfate in 55 cm3 of a 0.20 mol dm–3 solution of copper(II) sulfate.
40 cm3 of aqueous nitric acid of concentration 0.2 mol dm–3
ii 50 cm3 of calcium hydroxide solution of concentration 0.01 mol dm–3.
(Ar values: Cu = 63.5, O = 16.0, S = 32.1) Step 1 Change cm3 to dm3. 55 = _____ = 0.055 dm3 1000
Step 2 moles = concentration (mol dm–3) × volume of solution (dm3) = 0.20 × 0.055 = 0.011 mol Step 3 mass (g) = moles × M = 0.011 × (63.5 + 32.1 + (4 × 16.0))
15
= 1.8 g (to 2 significant figures)
CARRYING OUT A TITRATION A procedure called a titration is used to determine the amount of substance present in a solution of unknown concentration. There are several different kinds of titration. One of the commonest involves the exact neutralisation of an alkali by an acid (Figure 1.15). If we want to determine the concentration of a solution of sodium hydroxide we use the following procedure. ■ ■
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Get some of acid of known concentration. Fill a clean burette with the acid (after having washed the burette with a little of the acid). Record the initial burette reading. Measure a known volume of the alkali into a titration flask using a graduated (volumetric) pipette. Add an indicator solution to the alkali in the flask. Slowly add the acid from the burette to the flask, swirling the flask all the time until the indicator changes colour (the end-point).
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Record the final burette reading. The final reading minus the initial reading is called the titre. This first titre is normally known as a ‘rough’ value. Repeat this process, adding the acid drop by drop near the end-point. Repeat again, until you have two titres that are no more than 0.10 cm3 apart. Take the average of these two titre values.
Your results should be recorded in a table, looking like this: rough
1
2
3
final burette reading / cm3
37.60
38.65
36.40
34.75
initial burette reading / cm3
2.40
4.00
1.40
0.00
titre / cm3
35.20
34.65
35.00
34.75