AQA A Level Chemistry - Paper 3
Experiments
Acid-Base Titration - correct answer1. Fill a burette
2. Use a pipette to transfer 25cm³ of the other solution into the conical flask
3. Add 2-3 drops of indicator
4. Start adding from the burette, swirling constantly, until the indicator changes colour. Record how
much solution was added (rough titre)
5. Repeat to get concordant results and calculate a mean titre
Anti-Bumping Granules - correct answerSmall, rough pieces of silica that are added to the mixture
before reflux. They provide a rough surface on which small gas bubbles can grow, avoiding bumping.
They promote smooth, even boiling.
Checking Purity Of An Organic Solid - correct answer1. Place some solid in a melting point tube sealed at
one end
2. Place in m.p. apparatus and heat slowly
3. Record the melting temperature and the temperature when it stops melting
4. Repeat and average the remperatures
5. Compare m.p. to data book values
M.p. range of + or - 2°C = pure
Distillation - correct answerNB, oxidation: Dilute acid and less K2Cr2O7 than needed.
1. Heat liquid to boil and vaporise the most volatile component in the mixture. Anti-bumping granules
are used.
2. The vapour passes up from the flask and down into the condenser, where it is cooled by cold water
and ice, and condenses back to a liquid (the distillate) which is collected in the flask.
, 3. Any dissolved solids are left in solution because they have too high a boiling point to be distilled over .
Drying an Organic Liquid - correct answer1. Add a spatula of drying liquid (e.g. anhydrous CaCl2 or
MgSO4) to the organic liquid
2. Swirl
3. Add more of the drying agent until the liquid changes from cloudy to clear
4. Filter or decant off the liquid into a clean, dry flask
Extrapolation (Enthalpy Changes) - correct answer1. Measure 50g of water into polystyrene cup held in a
beaker
2. Start stop clock and record temp of water every minute for 3 minutes
3. On minute 4, add the 5g solid and stir vigorously
4. Read temp of solution every min for 15 mins
5. Plot graph of temp vs time
6. Extrapolate temp readings back from min 15 to min 4
7. Draw best fit line to give mean start temp from min 1 to min 3
8. Calculate temperature change at minute 4
How pH Changes When A Weak Acid Reacts With A Strong Base - correct answer1. Rinse pH probe
thoroughly with deionised water and shake gently. Place it in the standard pH 7 buffer solution to
calibrate.
2. Fill burette with 0.1M NaOH solution
3. Use pipette to transfer excactly 20cm³ of 0.1M ethanoic acid to a clean 100cm³ beaker
4. Rinse pH probe with deionised water and clamp it into the ethanoic acid solution in the beaker. Stir
gently with a rod and record the pH
5. Add the NaOH from the burette in 2cm³ intervals, stirring and measuring the pH after each interval
until 18cm³ is added
6. Add NaOH in 0.2cm³ intervals until 22cm³ is added, recording pH after each interval
7. Add NaOH in 2cm³ intervals until 40cm³ is added
8. Plot pH (y) against volume of NaOH added (x)
Experiments
Acid-Base Titration - correct answer1. Fill a burette
2. Use a pipette to transfer 25cm³ of the other solution into the conical flask
3. Add 2-3 drops of indicator
4. Start adding from the burette, swirling constantly, until the indicator changes colour. Record how
much solution was added (rough titre)
5. Repeat to get concordant results and calculate a mean titre
Anti-Bumping Granules - correct answerSmall, rough pieces of silica that are added to the mixture
before reflux. They provide a rough surface on which small gas bubbles can grow, avoiding bumping.
They promote smooth, even boiling.
Checking Purity Of An Organic Solid - correct answer1. Place some solid in a melting point tube sealed at
one end
2. Place in m.p. apparatus and heat slowly
3. Record the melting temperature and the temperature when it stops melting
4. Repeat and average the remperatures
5. Compare m.p. to data book values
M.p. range of + or - 2°C = pure
Distillation - correct answerNB, oxidation: Dilute acid and less K2Cr2O7 than needed.
1. Heat liquid to boil and vaporise the most volatile component in the mixture. Anti-bumping granules
are used.
2. The vapour passes up from the flask and down into the condenser, where it is cooled by cold water
and ice, and condenses back to a liquid (the distillate) which is collected in the flask.
, 3. Any dissolved solids are left in solution because they have too high a boiling point to be distilled over .
Drying an Organic Liquid - correct answer1. Add a spatula of drying liquid (e.g. anhydrous CaCl2 or
MgSO4) to the organic liquid
2. Swirl
3. Add more of the drying agent until the liquid changes from cloudy to clear
4. Filter or decant off the liquid into a clean, dry flask
Extrapolation (Enthalpy Changes) - correct answer1. Measure 50g of water into polystyrene cup held in a
beaker
2. Start stop clock and record temp of water every minute for 3 minutes
3. On minute 4, add the 5g solid and stir vigorously
4. Read temp of solution every min for 15 mins
5. Plot graph of temp vs time
6. Extrapolate temp readings back from min 15 to min 4
7. Draw best fit line to give mean start temp from min 1 to min 3
8. Calculate temperature change at minute 4
How pH Changes When A Weak Acid Reacts With A Strong Base - correct answer1. Rinse pH probe
thoroughly with deionised water and shake gently. Place it in the standard pH 7 buffer solution to
calibrate.
2. Fill burette with 0.1M NaOH solution
3. Use pipette to transfer excactly 20cm³ of 0.1M ethanoic acid to a clean 100cm³ beaker
4. Rinse pH probe with deionised water and clamp it into the ethanoic acid solution in the beaker. Stir
gently with a rod and record the pH
5. Add the NaOH from the burette in 2cm³ intervals, stirring and measuring the pH after each interval
until 18cm³ is added
6. Add NaOH in 0.2cm³ intervals until 22cm³ is added, recording pH after each interval
7. Add NaOH in 2cm³ intervals until 40cm³ is added
8. Plot pH (y) against volume of NaOH added (x)
