1.2 Amount of substance

Question 1

How much is 20 tonnes in kg and g

Answer 1

kg : 20000
g : 20000000

Question 2

Give equation for when iron reacts with excess chlorine and the product iron(iii) chloride is made

Answer 2

2Fe + 3Cl2 -> 2FeCl3

Question 3

How many atoms are there in 0.17g or NH3

Answer 3

Moles = 0.17 divided by Mr = 0.17 / 17 = 0.01
Number of molecules = moles x avogardos constat = 0.01 x 6.022 x 10^23 = 6.022 x 10^21
Number of atoms = number of molecules x atoms in NH3 = 6.022 x 10^21 x 4 = 2.408 x 10^22

Question 4

Calculate concentration in mol dm^3when 0.77g of MgCl2 are dissolved in water and made up to a volume of 25cm^3

Answer 4

moles = 0.77 / 95.3 = 8.0797 x10^-3
moles divided by (25 x10^-3) = 0.323 dm^3

Question 5

Sodium chloride is made by by dissolving 29.25g of NaCl in a beaker of water and making the volume up to 500cm^3. Calculate the concentration (in mol dm^-3) and convert the answer into g dm^-3

Answer 5

mr = 29.25 / 23 + 35.5 = 0.5
concentration = 0.5 divided by (500 x 10^-3) = 1 mol dm^-3
1mol dm^-3 x mr = 1 x 58.5 = 58.5 g dm^-3

Question 6

Student uses a funnel to fill the burette with sulfuric acid before starting the titration. After filling student forgets to remove the funnel from the top of the burette. Suggest why this may affect titre volume recorded

Answer 6

Additional drops of solution could have entered the burette from
the funnel making the burette level lower

Question 7

State an advantage of using a conical flask instead of a beaker for the titration

Answer 7

Less chance of splashing / less chance of losing any solution when swirling

Question 8

A pipette is used to add 25cm^3 of 0,112 mold dm^-3 aqeous sodium hydroxide to a conical flask. Uncertainty in using this pipette is +-0.06cm^3. Calculate the percentage uncertainty in using the pipette

Answer 8

(0.06 / 25) x 100 = 0.24%

Question 9

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 the air bubble was not there.
Explain why this air bubble increases the final burette reading of the rough
titration.

Answer 9

Some solution/acid replaces air bubble /
Solution/acid fills below the tap /
Air bubble takes up volume that would be filled by solution/acid

Question 10

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 10

Does not react (with the alkali) / does not change the number of moles (of
alkali)

Question 11

The student identified use of the burette as the largest source of uncertainty
in the experiment.
Using the same apparatus, suggest how the procedure could be improved
to reduce the percentage uncertainty in using the burette.
Justify your suggested improvement.

Answer 11

Use a larger mass of solid / or a more concentrated solution of the solid / or a more dilute solution of the acid
Justification: So a larger titre/reading will be needed OR larger volume of the acid

Question 12

Another student is required to make up 250 cm3 of an aqueous solution that
contains a known mass of MHCO3. The student is provided with a sample
bottle containing the MHCO3.
Describe the method, including apparatus and practical details, that the
student should use to prepare the solution. (6)

Answer 12

Stage 1 : transfers known mass of solid
a) Weigh the sample bottle containing the solid on a (2 dp)
balance
b) Transfer to beaker* and reweigh sample bottle
c) Record difference in mass
Stage 2: Dissolves in water
a) Add distilled / deionised water
b) Stir (with a glass rod) or swirl
c) Until all solid has dissolved
Stage 3 : Transfer, washing and agitation
a) Transfer to volumetric / graduated flask. Allow if a clear
description/diagram given eg long necked flask with 250 cm3
mark
b) With washings
c) Make up to 250 cm3
/ mark with water
d) Shakes/inverts/mixes

Question 13

The student rinses the burette with deionised water before filling with
sodium hydroxide solution.
State and explain the effect, if any, that this rinsing will have on the value of
the titre.

Answer 13

Titre value would increase / larger value
Because the sodium hydroxide solution would be more dilute

Question 14

Give meaning of term concordant titres

Answer 14

Titres that are within 0.1cm^3 of each other

Question 15

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 15

Idea that it ensures all / acid / sodium hydroxide / alkali /
reactants are in the mixture / solution / reaction or the idea that some of the
/ acid / sodium hydroxide / alkali / reactants would be on the
sides of the flask

Question 16

A teacher noticed that a student had not cleared a large air bubble from below
the burette tap in preparing the burette for use before starting the titration. This
air bubble was ejected during the first titration of the volumetric flask mixture.
State the effect that this mistake would have on the value of the first titre.
(b) State and explain the effect, if any, that this mistake would have on the
average titre for this experiment.

Answer 16

This would increase the titre value (from the real value of the titre)
b) No effect as the first titration is a trial as it can be ignored

Question 17

Suggest one other source of error in using the burette to carry out a
titration apart from droplets would affect titre reading

Answer 17

Air bubbles could be formed

Question 18

State why it is important to fill the space below the tap in the burette before beginning an accurate titration.

Answer 18

Space will fill during titration / titres or volumes added are too high

Question 19

Suggest one reason why it is important to wash the inside of the flask

Answer 19

To make sure all the solutions react with each other

Question 20

Washing with water decreases the concentration of the reagents in the
titration flask.
Suggest why washing with water does not affect the titre value.

Answer 20

Water does not change the number of moles of either reagent / reactant
or
Water is not a reagent / does not react with either reactant.

Question 21

Suggest one reason why repeating a titration makes the value of the
average titre more reliable.

Answer 21

Sinbge titration could be anamolous

Question 22

‘Take a clean 250 cm3 volumetric flask. Use the balance provided and a clean,
dry container, to weigh out the amount of solid required. Tip the solid into a clean,
dry 250 cm3 beaker and add about 100 cm3 of distilled water. Use a stirring rod to
help the solid dissolve, carefully breaking up any lumps of solid with the rod.
When the solid has dissolved, pour the solution into the flask using a filter funnel.
Add water to the flask until the level rises to the graduation mark.’
Suggest three further instructions that would improve the overall technique
in this account.

Answer 22

) Any three from:
A method of weighing by difference / wash the solid from its weighing
container into the beaker
If the nature of any washing is imprecise penalise once only.
Wash the (wet) rod into the flask / beaker after use
Do not allow a method where the solution is made up directly
in the flask.
Wash the (wet) beaker into the flask after transfer
Ignore any instructions that refer to rinsing equipment
(before use) or use of deionised water.
Wash the filter funnel (after transfer) into the flask
Use a teat pipette to make up to the mark on the volumetric flask
Ensure the bottom of the (liquid) meniscus is on the graduation mark
Mix / shake the final solution in the flask / invert flask

Question 23

What is 1dm^3 in m^3

Answer 23

0.001

Question 23

How much kelvin is zero degrees

Answer 23

273K

Question 23

A sample of 2.18 g of oxygen gas has a volume of 1870 cm^3 at a pressure of 101 kPa.
What is the temperature of the gas?
The gas constant is R = 8.31 J K–1 mol–1
.

Answer 23

Must use oxygen Mr as 32
Must also convert 1870 to m^3
T = 334K

Question 23

The student noticed that some of the liquid injected into the gas syringe did not vaporise.
Explain the effect that this has on the Mr calculated by the student.

Answer 23

Calculated Mr value would be greater than actual
Mr = mass / moles so dividing by too small a value of moles
gives a larger Mr than expected.

Question 23

Mass of fine needle syringe and contents
before injecting is 11.295g
Mass of fine needle syringe and contents after injecting is 10.835g
Each reading on the balance used to record the mass of the fine needle
syringe and contents had an uncertainty of ±0.001 g
Calculate the percentage uncertainty in the mass of liquid injected in this
experiment.

Answer 23

11.295 - 10.835 = 0.46g
2x0.001/0.46 x100 = 0.435%

Question 23

Suggest a reason, other than apparatus inaccuracy, why the Mr value determined from the experimental results differs from the actual Mr. Explain your answer.

Answer 23

1 The volume of the gas in the syringe is greater than the true volume
because some air leaked into the syringe

Question 24

Suggest, with a reason, an appropriate safety precaution that the student
should take when using toxic chloroalkane

Answer 24

Carry out in a fume cupboard
To avoid toxic vapour

Question 25

State meaning of term empirical formula

Answer 25

Simplest ratio of atoms of each element in a compound

Question 26

The pigment ’Cobalt Yellow’ contains an octahedral complex of cobalt(III) and nitrate(III) ions (NO2^-)
Cobalt Yellow contains 13.0% of
cobalt, 18.6% of nitrogen and 25.9% of potassium by mass. The remainder is oxygen.
(a) Use these data to calculate the empirical formula of Cobalt Yellow.
(b) Deduce the structural formula of the cobalt-containing ion in Cobalt Yellow

Answer 26

(a)CoN6K3O12
(b) Co(NO2)6^3-

Question 27

A student heated a solid sample of Na2CO3. xH2O for 1 minute to remove water and determine a value for x
Suggest a reason for the difference between the experimental value for x
and the correct value.
(b) Suggest how the procedure could be improved, using the same apparatus, to give a more accurate value for x
Justify your answer.
(c) Suggest how a student doing this experiment could check that all the water had been removed.

Answer 27

Failure to drive off all the water
OR
Failure to heat for long enough
OR
Not heated to constant mass
(b) Heat to constant mass / heat for longer / use a smaller mass
(c) Re heat the sample and check if mass is unchanged

Question 28

CH3CH2Br + KCN -> CH3CH2CN + KBr
Compound D is CH3CH2CN
Calculate the atom economy for compound D

Answer 28

Mr of CH3CH2Br + KCN = 174
Mr of compound D is 55
55/174 x 100 = 31.6%

Question 29

After reaction of some zinc metal with excess sulfuric acid, a student collected
40.8 g of ZnSO4.7H2O crystals. The yield of crystals was 70.0%.
What was the original mass of zinc used?

Answer 29

Mr of crystals = 287.5
40.8 x100/70 = 58.2857
Moles of zinc = 58.2857/287.5 = 0.2027
Mass of zinc = 65.4 x 0.2027 = 13.3g