Chemistry review Practice AQA A-level
Chemistry Questions with Verified
Answers
safety and hazards - Correct Answer - irritant: dilute acid and alkalis - wear goggles
- corrosive: stronger acids and alkalis - wear goggles
- flammable: keep away from naked flames
- toxic: wear gloves, avoid skin contact, and wash hands after use
- oxidizing: keep away from flammable/easily oxidized materials
hazardous substances in low concentrations or amounts will not pose the same risks as
the pure substance
heating in a crucible - Correct Answer method used for measuring mass loss in various
thermal decomposition reactions and also for mass gain when reacting magnesium in
oxygen
1.. weigh an empty clean dry crucible and lid
- the lid improves the accuracy of the experiment as it prevents loss of solid from the
crucible but should be loose fitting to allow gas to escape
- crucible needs to be dry - a wet crucible would give an inaccurate result (causes mass
loss to be too large as the water would be lost when heating)
2. add 2g of hydrated calcium sulfate to the crucible and weigh again
- large amounts (e.g. 50g) should not be used as the decomposition is likely to be
incomplete
- small amounts (0.100g) should not be used as the percentage uncertainties in
weighing will be too high
3. heat strongly with a Bunsen for a couple of minutes
4. allow to cool
5. weigh the crucible and contents again
6. heat crucible again and reweigh until you reach a constant mass (to ensure reaction
is complete)
using a gas syringe - Correct Answer - used for a variety of experiments where the
volume of a gas is measured (to work out moles of gas or to follow reaction rates)
- volume of a gas depends on pressure +
temperature = note both when recording volume
- moles of gas using ideal gas equation PV = north
,potential errors in using a gas syringe: - Correct Answer - gas escapes before bung
inserted
- syringe sticks
- some gases (e.g. CO2 or SO2) are soluble in water so the true amount of gas is not
measured
mark scheme:
1. the volume of the gas in the syringe (V) is greater than the true volume (because
some air leaked into the syringe)
- Mr. = m/n = m × RT/PV so if V is too large, Mr. is too small
2. the temperature measured (T) is less than the temperature of the gas in the syringe
(because the syringe heated faster than the oven and the oven temperature was not
constant)
- Mr. = m/n = m × RT/PV so if T is too small, Mr. is too small
3. the measured mass of liquid transferred to the syringe (m) is less than the actual
mass transferred
- Mr. = m/n = m × RT/PV so if m is too small, Mr. is too small
method for using a gas syringe to calculate the Mr. of propane - Correct Answer 1.
extract 0.20 cm3 of propane into a hypodermic syringe and then measure the mass of
this syringe
2. using hand protection, remove a gas syringe from the oven and note the volume of
air already in the
barrel - about 5 cm3
3. inject the propane through the self-seal cap into the barrel - the plunger will move
straight away.
4. put the gas syringe back into the oven
5. measure the mass of the empty hypodermic syringe immediately
6. after a few minutes measure the volume of the gas in the gas syringe, record the
temperature of the oven shelf and the pressure of the room
RP1: making a standard solution - Correct Answer 1. weigh the sample/weighing bottle
containing the required mass of solid on a 2 dip balance
- if using anhydrous sodium hydrogen sulfate make sure it is not too old as it will have
picked up water and therefore mass values will not be as accurate
2. transfer to beaker
3. reweigh empty sample/weighing bottle
4. record difference in mass
(or the known mass of solid in the weighing bottle can be transferred to beaker, washed,
and washings added to the beaker)
5. add 100cm3 of distilled water to the beaker and use a glass rod to stir to until all the
solid dissolves
,- if substance does not dissolve well in cold water the beaker + its contents could be
heated gently until all the solid dissolves
6. pour solution into a 250cm3 graduated/ volumetric flask via a funnel
- a graduated flask has one mark on the neck = level to fill to get accurate volume
- do not heat/put hot solution in flask (causes flask to expand and volume would be
incorrect)
7. rinse beaker and funnel (using distilled water) and add washings from the beaker and
glass rod to the volumetric flask
8. make up to the 250cm3 mark with distilled water using a dropping pipette for last few
drops
- make sure the bottom of the meniscus sits on line
- difficult to see meniscus with dark liquids (e.g. potassium manganite) - place white
paper behind
9. stopper and invert flask several times to ensure uniform solution
- shake volumetric flask thoroughly to ensure a uniform concentration
measuring mass accurately - Correct Answer 1. measure mass on 2 or 3d.p. balance of
a weighing bottle with the required quantity of solid in it
2. empty mass into reaction vessel/flask
3. reweigh the now empty weighing bottle
4. subtract mass of the empty weighing bottle from first reading to give exact of mass
actually added
diluting a solution - Correct Answer 1. pipette 25cm3 of original solution into a 250cm3
volumetric flask
- using a volumetric pipette is more accurate than a measuring cylinder as it has a
smaller uncertainty
2. make up to the mark with distilled water using a dropping pipette for last few drops
- use a teat pipette to make up to the mark in volumetric flask to ensure volume of
solution is accurately measured + does not go over the line
3. invert flask several times to ensure uniform solution
RP1: simple acid-base titration (titrations = to find conc. of a substance by reacting with
substance of known conc. = redox/neutralization) - Correct Answer 1. rinse the
equipment with the solutions they will contain (burette with acid, volumetric pipette with
alkali, conical flask with distilled water)
- if not rinsed: acid/alkali added may be diluted by residual water/react with substances
from previous titration = lower conc. of substance and larger titer
2. use a volumetric pipette to transfer 25cm3 of alkali (unknown conc.) into a 250cm3
conical flask + touch surface of alkali with pipette (to ensure correct amount is added)
- volumetric pipette = smaller uncertainty
- conical flask = easier to swirl without spillage
3. add standard/acid solution (known conc.) to burette
, - make sure jet space in the burette is filled with acid and does not contain air bubbles
(leads to errors: larger than expected titer reading)
- use funnel (at eye level) to prevent spillage
- don't leave the funnel in the burette as small drops of liquid may fall from the funnel
during titration (leads to a false burette reading (gives a lower titer volume))
4. add a few drops of indicator to conical flask and refer to colour change at end point
- only a few drops: generally weak acids so too much will affect titration result
- phenolphthalein (strong alkali e.g. Noah): colorless in acid, pink in alkali, endpoint =
colorless <=> pink
- methyl orange (weak alkali e.g. NH3): red in acid, yellow in alkali, endpoint = orange
- use a white tile underneath the flask to help observe the colour change
5. add acid to alkali slowly whilst swirling the mixture and add acid drop wise near
endpoint (until indicator undergoes a definite, permanent colour change)
6. note burette reading before + after addition of acid
- make sure all burette readings are to the appropriate precision and are read from the
bottom of the meniscus
- note titer volume (change in volume in the burette) to 2 dip (0.05cm3)
7. rep
wash sides of flask with distilled water - Correct Answer - distilled water can be added to
the conical flask during a titration to wash the sides of the flask so that all the acid on
the side is washed into the reaction
mixture to react with the alkali
- does not affect the titration reading as water does not react with the reagents or
change the number of moles of reagents/acid added
safety precautions titrations - Correct Answer - acids and alkalis are corrosive (at low
concentrations acids are irritants)
- wear eye protection and gloves
- if spilled immediately wash affected parts after spillage
- if substance is unknown treat it as potentially toxic and wear gloves
manganite redox titrations - Correct Answer redox titration between Fe2+ with MnO4-
(purple):
MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)
- self-indicating (significant colour change from reactant to product): colorless → purple
- if manganite is in the burette then the endpoint of the titration will be the first
permanent pink colour
- purple colour of manganite can make it difficult to see the bottom of meniscus in the
burette
choosing correct acid (to supply the 8H+ ions):
- only dilute sulfuric acid is used for manganite titrations
Chemistry Questions with Verified
Answers
safety and hazards - Correct Answer - irritant: dilute acid and alkalis - wear goggles
- corrosive: stronger acids and alkalis - wear goggles
- flammable: keep away from naked flames
- toxic: wear gloves, avoid skin contact, and wash hands after use
- oxidizing: keep away from flammable/easily oxidized materials
hazardous substances in low concentrations or amounts will not pose the same risks as
the pure substance
heating in a crucible - Correct Answer method used for measuring mass loss in various
thermal decomposition reactions and also for mass gain when reacting magnesium in
oxygen
1.. weigh an empty clean dry crucible and lid
- the lid improves the accuracy of the experiment as it prevents loss of solid from the
crucible but should be loose fitting to allow gas to escape
- crucible needs to be dry - a wet crucible would give an inaccurate result (causes mass
loss to be too large as the water would be lost when heating)
2. add 2g of hydrated calcium sulfate to the crucible and weigh again
- large amounts (e.g. 50g) should not be used as the decomposition is likely to be
incomplete
- small amounts (0.100g) should not be used as the percentage uncertainties in
weighing will be too high
3. heat strongly with a Bunsen for a couple of minutes
4. allow to cool
5. weigh the crucible and contents again
6. heat crucible again and reweigh until you reach a constant mass (to ensure reaction
is complete)
using a gas syringe - Correct Answer - used for a variety of experiments where the
volume of a gas is measured (to work out moles of gas or to follow reaction rates)
- volume of a gas depends on pressure +
temperature = note both when recording volume
- moles of gas using ideal gas equation PV = north
,potential errors in using a gas syringe: - Correct Answer - gas escapes before bung
inserted
- syringe sticks
- some gases (e.g. CO2 or SO2) are soluble in water so the true amount of gas is not
measured
mark scheme:
1. the volume of the gas in the syringe (V) is greater than the true volume (because
some air leaked into the syringe)
- Mr. = m/n = m × RT/PV so if V is too large, Mr. is too small
2. the temperature measured (T) is less than the temperature of the gas in the syringe
(because the syringe heated faster than the oven and the oven temperature was not
constant)
- Mr. = m/n = m × RT/PV so if T is too small, Mr. is too small
3. the measured mass of liquid transferred to the syringe (m) is less than the actual
mass transferred
- Mr. = m/n = m × RT/PV so if m is too small, Mr. is too small
method for using a gas syringe to calculate the Mr. of propane - Correct Answer 1.
extract 0.20 cm3 of propane into a hypodermic syringe and then measure the mass of
this syringe
2. using hand protection, remove a gas syringe from the oven and note the volume of
air already in the
barrel - about 5 cm3
3. inject the propane through the self-seal cap into the barrel - the plunger will move
straight away.
4. put the gas syringe back into the oven
5. measure the mass of the empty hypodermic syringe immediately
6. after a few minutes measure the volume of the gas in the gas syringe, record the
temperature of the oven shelf and the pressure of the room
RP1: making a standard solution - Correct Answer 1. weigh the sample/weighing bottle
containing the required mass of solid on a 2 dip balance
- if using anhydrous sodium hydrogen sulfate make sure it is not too old as it will have
picked up water and therefore mass values will not be as accurate
2. transfer to beaker
3. reweigh empty sample/weighing bottle
4. record difference in mass
(or the known mass of solid in the weighing bottle can be transferred to beaker, washed,
and washings added to the beaker)
5. add 100cm3 of distilled water to the beaker and use a glass rod to stir to until all the
solid dissolves
,- if substance does not dissolve well in cold water the beaker + its contents could be
heated gently until all the solid dissolves
6. pour solution into a 250cm3 graduated/ volumetric flask via a funnel
- a graduated flask has one mark on the neck = level to fill to get accurate volume
- do not heat/put hot solution in flask (causes flask to expand and volume would be
incorrect)
7. rinse beaker and funnel (using distilled water) and add washings from the beaker and
glass rod to the volumetric flask
8. make up to the 250cm3 mark with distilled water using a dropping pipette for last few
drops
- make sure the bottom of the meniscus sits on line
- difficult to see meniscus with dark liquids (e.g. potassium manganite) - place white
paper behind
9. stopper and invert flask several times to ensure uniform solution
- shake volumetric flask thoroughly to ensure a uniform concentration
measuring mass accurately - Correct Answer 1. measure mass on 2 or 3d.p. balance of
a weighing bottle with the required quantity of solid in it
2. empty mass into reaction vessel/flask
3. reweigh the now empty weighing bottle
4. subtract mass of the empty weighing bottle from first reading to give exact of mass
actually added
diluting a solution - Correct Answer 1. pipette 25cm3 of original solution into a 250cm3
volumetric flask
- using a volumetric pipette is more accurate than a measuring cylinder as it has a
smaller uncertainty
2. make up to the mark with distilled water using a dropping pipette for last few drops
- use a teat pipette to make up to the mark in volumetric flask to ensure volume of
solution is accurately measured + does not go over the line
3. invert flask several times to ensure uniform solution
RP1: simple acid-base titration (titrations = to find conc. of a substance by reacting with
substance of known conc. = redox/neutralization) - Correct Answer 1. rinse the
equipment with the solutions they will contain (burette with acid, volumetric pipette with
alkali, conical flask with distilled water)
- if not rinsed: acid/alkali added may be diluted by residual water/react with substances
from previous titration = lower conc. of substance and larger titer
2. use a volumetric pipette to transfer 25cm3 of alkali (unknown conc.) into a 250cm3
conical flask + touch surface of alkali with pipette (to ensure correct amount is added)
- volumetric pipette = smaller uncertainty
- conical flask = easier to swirl without spillage
3. add standard/acid solution (known conc.) to burette
, - make sure jet space in the burette is filled with acid and does not contain air bubbles
(leads to errors: larger than expected titer reading)
- use funnel (at eye level) to prevent spillage
- don't leave the funnel in the burette as small drops of liquid may fall from the funnel
during titration (leads to a false burette reading (gives a lower titer volume))
4. add a few drops of indicator to conical flask and refer to colour change at end point
- only a few drops: generally weak acids so too much will affect titration result
- phenolphthalein (strong alkali e.g. Noah): colorless in acid, pink in alkali, endpoint =
colorless <=> pink
- methyl orange (weak alkali e.g. NH3): red in acid, yellow in alkali, endpoint = orange
- use a white tile underneath the flask to help observe the colour change
5. add acid to alkali slowly whilst swirling the mixture and add acid drop wise near
endpoint (until indicator undergoes a definite, permanent colour change)
6. note burette reading before + after addition of acid
- make sure all burette readings are to the appropriate precision and are read from the
bottom of the meniscus
- note titer volume (change in volume in the burette) to 2 dip (0.05cm3)
7. rep
wash sides of flask with distilled water - Correct Answer - distilled water can be added to
the conical flask during a titration to wash the sides of the flask so that all the acid on
the side is washed into the reaction
mixture to react with the alkali
- does not affect the titration reading as water does not react with the reagents or
change the number of moles of reagents/acid added
safety precautions titrations - Correct Answer - acids and alkalis are corrosive (at low
concentrations acids are irritants)
- wear eye protection and gloves
- if spilled immediately wash affected parts after spillage
- if substance is unknown treat it as potentially toxic and wear gloves
manganite redox titrations - Correct Answer redox titration between Fe2+ with MnO4-
(purple):
MnO4-(aq) + 8H+(aq) + 5Fe2+(aq) → Mn2+(aq) + 4H2O(l) + 5Fe3+(aq)
- self-indicating (significant colour change from reactant to product): colorless → purple
- if manganite is in the burette then the endpoint of the titration will be the first
permanent pink colour
- purple colour of manganite can make it difficult to see the bottom of meniscus in the
burette
choosing correct acid (to supply the 8H+ ions):
- only dilute sulfuric acid is used for manganite titrations
