Helen Spencer, candidate number: 9326, centre number: 55245
Chemistry Practical Test (Part A): Planning Exercise
Testing unknown solutions
In this practical investigation, I have to devise a logical sequence of chemical reactions to find out which unknown chemical goes with which name. I then have to outline a titration procedure for working out the exact concentration of aqueous nitric acid using aqueous sodium carbonate, concentration of 0.500 mol dm¯³
Precautions – Health and Safety
- Wear safety glasses
- Handle all chemicals with care
- Wash hands before and after handling and chemicals
- Have an eye wash available.
Calcium carbonate (marble chips),
Dilute hydrochloric acid,
Aqueous ammonia solution,
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Delivery tube and stopper,
Test tube holder
- Testing for calcium hydroxide:
Carbon dioxide bubbled through calcium hydroxide gives a milky white precipitate, calcium carbonate.
Ca(OH)2 (aq) + CO2 (g) CaCO3 (s) + H2O (l)
Calcium hydroxide + carbon dioxide calcium carbonate + water
- testing for potassium bromide and sodium chloride:
- Both bromide and chloride are halide ions
- They both react with silver ions to form a coloured precipitate
Cl¯ present – silver chloride forms – white precipitate.
Br¯ present – silver bromide forms – cream precipitate.
- Colour identification not completely reliable – add ammonia solution
- If chloride ion present, solution turns colourless
- If bromide ion present, solution goes slightly clearer.
- Positive test for acids:
Acid + carbonate carbon dioxide + water + salt
- If both of the two unknown solutions left show a positive result, they are both acids, so I only need to work out either which is the nitric or the ethanoic acid, because the last solution would be the other acid.
- Test for ethanoic acid:
Esterification – if solution is ethanoic acid, it smells like pears.
- Heat gently, allow to cool, smell cautiously
Alcohol + carboxylic acid ester + water
Danger – adding sulphuric acid to a dilute solution means that it could react with the water – could be explosive
Risk assessment of Nitric acid
- Oxidizing agent
- Dangerous with ethanoic acid – if comes into contact with, may cause fire
- Don’t inhale fumes – makes last experiment not recommendable
- Wear eye protection
- Vapour dangerous
Titration for concentration of nitric acid
Graduated pipette filler
250cm³ conical flask
Stand and clamp
- Rinse burette with alkali, fill with same solution
- Record initial reading
- Use pipette filler, rinse pipette with nitric acid, fill with same solution to 25cm³ mark, transfer to conical flask.
- Add 2/3 drops of indicator
- Run alkali from burette whilst swirling conical flask until colour goes from yellow to blue
- Record reading of burette to nearest 0.05cm³
- Repeat experiment until you have three readings within 0.1cm³ of each other, excluding trial.
- Work out mean titre (for calculation = X)
Solution A – nitric acid,
volume – 25.0cm³,
concentration – ?
Solution B – sodium carbonate,
volume – X,
concentration – 0.500 mol dm¯³
1000cm³ of solution B = 0.5 mols NaCO3
In Xcm³ NaCO3 there are: 0.5X / 1000 mol
NaCO3 + HNO3
1 mol NaCO3 reacts with 1 mol HNO3
Mols HNO3 present in 1000cm³ = (0.5X /1000) x 1000
= 0.5X ÷ 25
The concentration of the nitric acid solution is therefore 0.5X ÷ 25 mol.dm¯³
Fair test, precise and reliable data
- Will repeat all experiments 3 times
- All experiments – room temperature
- Will use accurate equipment, e.g. calibrated pipette filler
- Will compare data against specimen data to check reliability.