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Analysis and Synthesis
C3.4…read more

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C3.4.1 Tests for positive ions
Positive ions can be identified using flame tests or sodium hydroxide solution
The hydroxides of most 2+ and 3+ ions are insoluble in water
When sodium hydroxide is added to solutions of these ions a precipitate of the metal
hydroxide forms
Aluminium, calcium and magnesium ions form white precipitate
When excess sodium hydroxide solution is added the aluminium hydroxide precipitate
solution dissolves
Copper (II) hydroxide precipitate is blue Metal ion Flame colour
Iron (II) hydroxide precipitate is green Lithium (Li+) Crimson (Red)
Sodium (Na+) Yellow
Iron (III) hydroxide precipitate is brown
Potassium (K+) Lilac
Calcium (Ca2+) Red
Barium (Ba2+) Green…read more

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C3.4.2 Tests for negative ions
There are three tests for negative ions
Carbonate ions: Add dilute hydrochloric acid to the substance to see if it fizzes. If it
does and the gas turns limewater milky, the substance contains carbonate ions
Halide ions: Add dilute nitric acid and then silver nitrate solution
Chloride ions give a white precipitate
Bromide ions give a cream precipitate
Iodide ions give a yellow precipitate
Sulphate ions: Add dilute hydrochloric acid and then barium chloride solution. If a
white precipitate forms, sulphate ions are present…read more

Slide 4

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C3.4.3 Titrations
A titration is used to measure accurately how much acid and alkali react
completely together
A pipette is used to accurately measure the volume of alkali put into a conical
flask then an indicator is added to this
A burette is filled with acid which is gradually added
When the indicator changes colour the end point has been reached.
The volume of acid used is found from the initial and final burette reading
Titration should be done several times for more accurate and reliable results…read more

Slide 5

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C3.4.4 Titration calculations
Concentrations of solutions are measured in g/dm3 or mol/dm3
If we know the mass or the number of moles of a substance dissolved in a given volume of
solution we can calculate its concentration
If we know the volume of a solution and its concentration we can calculate the mass or the
number of moles of the substance in any volume of solution
Worked example
50cm3 of solution was made using 5.6g of potassium hydroxide, KOH. What's its
1cm3 of solution contains (5.6/50)g so 1dm3 of solution contains (5.6/50)x1000g=112g
Concentration of solution= 112g/dm3
1 mole KOH=(39+16+1)=56g 112g/56g=2 mole Concentration of solution= 2mol/dm3…read more

Slide 6

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C3.4.4 Titration calculations
Titrations are used to find the volumes of solutions that react exactly
The concentrations are calculated using balanced symbol equations and moles
Worked example
A student found that 25.0cm3 of sodium hydroxide solution with an unknown concentration reacted with
exactly 20.0cm3 of 0.50mol/dm3 hydrochloric acid. What was the concentration of the sodium hydroxide
The equation for this reaction is: NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)
The concentration of the HCl is 0.50mol/dm3, so 0.50mol of HCl are dissolved in 1000cm3 of acid.
Therefore 20.0cm3 of acid contains 20 x 0.50/1000mol = 0.010mol HCl
The equation for the reaction tells us that 0.010mol of HCl will react exactly with 0.010mol of NaOH.
This means that there must be 0.010 moles of NaOH in the 25.0cm3 of solution in the conical flask.
So, the concentration of NaOH solution = (0.010/25) x 1000 = 0.40mol/dm3…read more

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