Practical Questions

Question 1

How to remove soluble impurities

Answer 1

Wash with or add deionised water or wash with a small amount of cold water

Question 2

Explain why different observations are made when aq barium chloride is added seperately to aq magnesium sulfate and to aq magnesium nitrate

Answer 2

As baso4 is insoluble and no reaction occurs so it remains in a solution so the reacs differ

Question 3

You are given a sample of saturated calcium hydroxide solution, outline the practical steps that you would take to determine the solubility of calcium hydroxide in this sol

Answer 3

Take a known vol of the saturated solution,
Evaporate the filter to dryness
Weigh the residue

Question 4

The student used a wash bottle containing deionised water when approaching the end point to rinse the inside of the conical flask, explain why this improved the accuracy of the titration

Answer 4

Ensures the acid wasnt left behind to react and give invalid results

Question 5

What does concordant titres mean

Answer 5

They are within 0.1cm cubed of each other

Question 6

The mass of mgo obtained in this exp is slightly less than expected from the mass of mgno3 used suggest one practical reason why

Answer 6

As some of the solid may have been lost for example blown away with the gas

Question 7

Suggest a reason other than apparatus inaccuracy for why the mr is smaller when experimental

Answer 7

As the vol of gas may be greater than the true value as some air may have leaked into the syringe causing an increase in the mass so mr decreases

Question 8

Safety precaution needed for toxic gases

Answer 8

Carry out the experiments in a fume cupboard to avoid toxic vapour

Question 9

The student rinsed the burette before filling it w sodium hydroxide. Explain why the student should use the sodium hydroxide sol rather than the water

Answer 9

As water would dilute the sodium hydroxide

Question 10

If we make hcl in a reac or smth and it asks how the company could profit

Answer 10

Just say by selling the hcl

Question 11

strontium carbonate reacts with nitric acid to produce a solution of strontium nitrate.
Strontium sulfate does not react w nitric acid describe briefly how you could obtain strontium sulfate from a mix of strontium carbonate and strontium sulfate

Answer 11

Add nitric acid to the mixture until in excess

And filter to isolate strontium sulfate

Question 12

How to sep and purify smth

Answer 12

Filter off remaining ppt, wash to remove soluble impurities, dry to remove water

Question 13

Pure mg reacts completely w an excess of dilute h2so4 and is v rapid initially. This reac slows down and stops before all the calcium has reacted. Use your knowledge of gp 2 sulfates to explain why these reacs w h2so4 are so diff

Answer 13

Bc magnesium sulfate is soluble and calcium sulfate is insoluble. The calcium is coated with sulfate which prevents further reac w the acid

Question 14

Why do anti bumping granules prevent bumping

Answer 14

As they form smaller bubbles preventing larger ones to be formed

Question 15

give two ways of maximising the yield of propanal obtained by distillation of the reac mixture

Answer 15

Keep the temp of the reac mixture bellow the bp of propan-1-ol

Cool the distillate

Question 16

explain why a mixture could be filtered before the addition of something

Answer 16

to remove excess solid

Question 17

why would something be washed before it was dried

Answer 17

to remove excess acid`

Question 18

how would an average titre change if a few cm rather than a few drops of the indicator was used by mistake

Answer 18

decrease, bc less acid would be needed

Question 19

give one reason why a 250cm cubed conical flask is preferred to a beaker

Answer 19

bc less chance of losing liquid when swirling

Question 20

suggest one reason why calcium hydroxide sol is not used in the titration of a 0.200 mol dm-3 solution of an acid

Answer 20

as dilution of the acid is needed

Question 21

describe a method for increasing accuracy calorimetry

Answer 21

insulate beaker to reduce heat loss, stirring, record temp for a suitable amount of time to have an accurate initial temperature, plot the results on a graph, get the temp change at the 4th minute extroplate cooling

Question 22

the mass of x obtained in the experimetn is slightly less than expected, suggest one practical reason for this

Answer 22

solid lost in weighing

Question 23

the rates of hydrolysis of two chloroalkanes can be investigated aqueous silver nitrate to the chloroalkanes. outline a method to compare the rate of hydrolysis of 1-chlorobutane with 2-chlorobutane. state how it would involve a fair test

Answer 23

add the same volume of agno3 in each experiment, use the same temp and record the time to measure a sensible observation of the amount of agcl precipitate formed. and the rate is the amount formed/ time taken. less time taken and more product means a faster rate

Question 24

before adding the solution from the burette in the rough titration there was an air bubble below the tap. at the end of this titration it wasnt there. explain why this air bubble increases the final burette reading of the rough titration

Answer 24

as the air bubble would take up volume that would have been filled by the solution

Question 25

during the titration the student washed the inside of the conical flask with some distilled water. suggest why this washing does not give an incorrect result

Answer 25

as it does not react with the alkali

Question 26

the student identified the use of the burette as the largest source of uncertainity in the experiment. using the same apparatus suggest how the procedure could be improved to reduce the percentage uncertainity in using the burette

Answer 26

use a larger mass of the solid so a larger titre would be needed

Question 27

another student is required to make 250cmcubed of an aq solution that contains a known mass of mhco3. the student is provided with a sample bottle containing the mhco3. describe practical they should use to prepare the sol (6 marks)

Answer 27

weigh sample bottle containing the solid on a balance, transfer to beaker and reweigh sample bottle record the differences in mass
add distilled water stir with a glass rod to ensure that the solid dissolves
transfer to graduated flask, with washings,make up 250cm cubed and shake it or invert it.

Question 28

a solution of sodium chlorate was added to a colourless solution of potassium iodide. suggest what is observed

Answer 28

goes brown due to iodine because iodine is oxidised

Question 29

suggest one reason other than incomplete combustion or heat transfer to the atmosphere why the students value for the enthalpy of combustion of methanol is different from that in a data book- 3 points

Answer 29

experiment not done under standard conditions
evaporation of alcohol
thermometer not being taken into account

Question 30

suggest one reason other than use of mean bond enthalpies why a value for the enthalpy of combustion of a liquid alkane is different from the value obtained using the expression (liquid alkane vs gaseous)

Answer 30

as the alkane is not gaseous

Question 31

the student used a wash bottle containing deionised water when approaching the end-point to rinse the inside of the conical flask. explain why this improved the accuracy of the titration

Answer 31

ensures that all the acid remained in the mixture

Question 32

how does the contamination occur whereby ethanol turned into ethanoic acid

Answer 32

oxidation

Question 33

the student failed to fill the burette correctly so that the gap between the tip of the burette still contained air. suggest what effect this would have on the measured vol of bromine water in this trial

Answer 33

measured volume higher, level in burette falls as tap is filled before the liquid is delivered.

Question 34

how could the student improve the titration w drops

Answer 34

choose a larger vol of oil
dissolve it in organic solvent
transfer this to a flask and make up to 250cm cubed with solvent
titrate 25cm cubed samples from the flask

Question 35

suggest a reason for the diff between the exp value for x and the correct value

Answer 35

failute to heat to a constant mass or failure to evaporate all the water

Question 36

suggest how the procedure could be improved, using the same apparatus to give a more accurate value for x

Answer 36

heat for longer, ensure all the water has been driven off

Question 37

some of the liquid injected did not evaporate because it dripped into the gas synringe nozzle outside the oven. explain how this would affect the value of the mr of y

Answer 37

lower volume, therefore mr would be greater

Question 38

suggest one possible danger when a metal carbonate is reacted with an acid in a sealed flask

Answer 38

the pressure may build up from gases leading to an explosion

Question 39

if one reactant goes to make products, why could the yield be less than expected

Answer 39

decomposition could not be complete, or loss of a gas

Question 40

suggest one reaosn why it is hard to obtain a pure sample of no2 from a reac producing no2 pbo and o2

Answer 40

bc it may be hard to seperate the no2 from the o2

Question 41

how to measure rate in an experiment

Answer 41

the change in ph of a reaction. (ph meter can be used to measure this at regular intervals)
amount of heat lost (measure mass lost as gas lost, moles can then be used to work out gas lost and reactants left)
volume of gas produced (Using gas syringe, ideal gas equation to wrok out moles of gas produced= molar ratio for reactant concs)
colour change then using a colorimeter or cross which is subjective

Question 42

colorimeter

Answer 42

measure of absorbance of light

more conc a sample is, the darker the colour and hence more light is absorbed.

have to plot a calibration curve, then we can use that and absorbance to find out the conc of iodine at that point

Question 43

The student purified the crude solid product, N−phenylethanamide, by
recrystallisation.
(i) Outline the method that the student should use for this recrystallisation

Answer 43

Dissolve the product in the minimum volume of water / solvent
(in a boiling tube / beaker)
If dissolving is not mentioned, CE = 0 / 4
1
Hot water / solvent
Steps must be in a logical order to score all 4 marks
1
Allow the solution to cool and allow crystals to form.
1
Filter off the pure product under reduced pressure / using a
Buchner funnel and side arm flask

Question 44

Outline how you would carry out a simple laboratory process to show that the
recrystallised product is a pure sample of N−phenylethanamide.

Answer 44

Measure the melting point
1
Use of melting point apparatus or oil bath
1
Sharp melting point / melting point matches data source value

Question 45

Assume that the reaction goes to completion.
Suggest two practical reasons why the percentage yield for this reaction may
not be 100%

Answer 45

Product left in the beaker or glassware
Sample was still wet
Sample lost during recrystallisation

Question 46

You are provided with a small sample of pure aspirin in a melting point tube.
Describe briefly how you would determine an accurate value for the melting point of
aspirin

Answer 46

heat melting tube in an oil bath

slowly near the melting point

Question 47

State two observations, during this melting point determination, that would indicate
that the sample is not pure.

Answer 47

wide melting range

below the true mp

Question 48

Suggest why a pure sample of aspirin may sometimes appear to melt at a temperature different from 135 °C.

Answer 48

Temperature on thermometer not the same as the sample

Question 49

Step 1: The sample is dissolved in a minimum volume of hot water.
Step 2: The solution is filtered hot.
Step 3: The filtrate is cooled in ice to form crystals.
Step 4: The crystals are collected by filtration, washed with cold water and left to dry.
Explain the purpose of each underlined point.
Minimum volume ………………………………………………………………………………………..
…………………………………………………………………………………………………………………
Hot water ………………………………………………………………………………………………….
…………………………………………………………………………………………………………………
Filtered hot ………………………………………………………………………………………………..
…………………………………………………………………………………………………………………
Cooled in ice ……………………………………………………………………………………………..
…………………………………………………………………………………………………………………
Washed with cold water

Answer 49

Minimum volume and hot water:
Any two from:
to obtain saturated solution
to increase yield / reduce amount left in solution
enable crystallisation (on cooling)

Filtered hot: to remove insoluble impurities

Cooled in ice: to increase amount of crystals that are formed

Washed with cold water: to remove soluble impurities

Question 50

Give one reason why using a water bath is better than direct heating with a Bunsen
burner.

Answer 50

alcohol is flammable