p5

Question 1

Explain why the FIRST sample of gas collected from either apparatus should not be used for the Mr determination of gas X

Answer 1

• the sample will contain air from the apparatus

Question 2

Suggest why is it necessary for the flask to be stoppered

Answer 2

to prevent CO2 from entering the flask

Question 3

Suggest why the student leaves the flask of Ca(OH)2 + water stoppered for 24h

Answer 3

to ensure that the solution is saturated

Question 4

It is more appropriate for the time to be recorded to the nearest second.

Suggest 1 reason for this.

Answer 4

reaction time of observer is too slow to measure up to 0.01

of a second

Question 5

The student uses the results to obtain an average of CO2 produced for each ore and records it in the table

Suggest another reason why each ore is tested 3 times

Answer 5

to improve reliability of results

Question 6

The mass change from the thermal decomposition of a ground-up sample of rhodochrosite could be used to determine the % of carbonate in the sample. The ground-up sample is strongly heated in a cruicble.

ii) State the measurements the students should make to determine the % of manganese carbonate is in the sam
iii) explain how students can ensure the results are as accurate as possible

Answer 6

ii) - Mass of crucible
- Mass of crucible and ore before heating
- Mass of crucible and ore after heating

iii) heat to constant mass to ensure reaction is completed

Question 7

The student rinsed a burette with solution A before filling it with solution A.
Explain why this improves the accuracy of the results

Answer 7

• to ensure that there is no water in burette to dilute the
  solution when the burette is filled
• and solution in the burette is at expected concentration.

Question 8

Suggest 1 major adv of filtering the mixture under reduced pressure compared with filtering using gravity

Answer 8

increased rate of filtration

Question 9

The student rinsed the residue, transferred it to a crucible and placed it in a warm oven to dry it.

i) what shld the student do to ensure that the drying
process is complete

Answer 9

heat to constant mass

Question 10

State and explain how the no. of moles of AgCl formed would change if the student used tap water instead of distilled water to dissolve the grp 2 chloride

Answer 10

Moles of AgCl will ↑ bcuz tap water contains chloride ions

Question 11

Rough 1 2
Titre 21.20 21.50 22.25

Identify the problem with the student’s titration method and suggest how it could be improved

Answer 11

• Titre values are not concordant (consistent)

- repeat titration until concordant titres are obtained

Question 12

Mixture of HNO3 and HCL is passed thru capillary viscometer. The capillary viscometer is rinsed first with water, and then passed with propanone.

Suggest why the capillary is rinsed viscometer is rinsed with water and then propanone

Answer 12

water - to remove remaining HNO3 and HCL

propanone - to remove water from the tube

Question 13

Heating under reflux process is used instead of regular heating. Explain the benefit of this

Answer 13

• ↑ rate of reaction

- Prevents vapour loss

Question 14

Student suggests that placing a bung in the top of condenser would improve the experiment.
Comment on whether the student is correct

Answer 14

• Incorrect

- bcuz heating causes pressure to build up in the apparatus, this would lead to apparatus popping

Question 15

At (1atm), distilled water is know to boil at 100.00 °C.

Suggest why the bp of distilled in this experiment was found to be 99.48 °C

Answer 15

• The pressure in laboratory is lower than at sea level /

1atm

Question 16

Explain why it is important for DRY propanone to be used as a solvent

Answer 16

to prevent reaction w water if wet

Question 17

The reaction btw CH3CH2CH2Br and NaI proceeds via an SN2 mechanism. This experiment is repeated using an isomer of CH3CH2CH2Br that reacts via Sn1 and SN2.

State and Explain whether the student will be able to chow that the rate of this reaction is prop to the conc of NaI.

Answer 17

• No,

- bcuz rate of SN1 is only dependent on the conc of the organic compound, which is not changed for this experiemnt

Question 18

Suggest why diluting a more conc solution is a more accurate method of preparing this solution than by dissolving urea in 50.0cm3 of water

Answer 18

• Mass of urea is too small to measure accurately

Question 19

Na2CO3 + HNO3

State how the student would know there was no more acid present in the mixture.

Answer 19

no more effervescence seen

Question 20

Outline a titration method with aqueous iodine against Na2S2O3 (in burette)

Answer 20

• Rinse the burette w Na2S2O3, ensure this runs through below the tap of burette
• Now, w the tap cosed, pour the Na2S2O3 into burette at eye level, remove air bubbles by running some of the Na2S2O3 through the tap.
• Fill Na2S2O3 to 0.00cm3 mark

Question 21

State 1 improvement to reduce heat loss in beaker

Answer 21

• use polystyrene cup instead of glass beaker
• cover beaker w a lid
• insulate the glass beaker

Question 22

name the apparatus to heat gas syringe at diff temperatures btw 295K and 370K so that V reading of gas syringe can easily be measured.

Answer 22

• thermostatically controlled water bath

Question 23

Lowering [Mnz+] causes the value of the electrode potential of the maganese half-cell to become more negative. Suggest why this happens

Answer 23

• Mn –> Mn^z+ + 2e
• By ↓ {Mn^z+], equilibrium will shift to the RHS
• ↑ ions produced
• tendency to oxidise ↑