Gay Lussac’s of combining volumes states that gases react in simple ratio with one another and to volumes of the products provided that temperature and pressure remain constant. In this article, you will understand how to apply this law in calculation by studying the following examples below: 1. 2H 2 + O 2 → H 2 O In the reaction above, what volume of hydrogen would be left over when 300cm 3 of oxygen and hydrogen are exploded in a sealed tube? 1cm 3 of oxygen = 2cm 3 of hydrogen 300cm 3 of oxygen = 2 x 300 = 600cm 3 Volume of left over = 1000 – 600 = 400cm 3 2. Calculate the volume of carbon (II) oxide required to react with 40cm 3 of oxygen. 2CO + O 2 → 2CO 2 1cm 3 of oxygen = 2cm 3 of CO 40cm 3 of oxygen = 2 x 40 = 80cm 3 3. Calculate the volume of residual gases that would be produced when 100cm 3 of sulphur (IV) oxide reacts with 20cm 3 of oxygen 2SO 2 + O 2 → 2SO 3 1cm 3 of O 2 = 2cm 3 20cm 3 of O 2 = 2 x 20 = 40cm 3
Paper 1
1. All your burette reading (initial and final) as well
as size of the pipette must be recorded, but no experimental procedure is
required. All calculations must be done on this question paper.
A is a solution containing 0.15moldm-3 HNO3.
B is a solution containing 4.20gdm-3 of X2CO3
salt.
Put A into
the burette and titrate against 20cm3 or 25cm3 portion of
B using methyl orange as an
indicator. Repeat the titration three more times and record your results in the
table below.
Volume of pipette used ……….cm3
|
Titration |
Rough |
1 |
2 |
3 |
|
Final burette reading(cm3) |
|
|
|
|
|
Initial burette reading (cm3) |
|
|
|
|
|
Volume of A
used(cm3) |
|
|
|
|
(6 marks)
a) Calculate the average volume of A used.
(2 marks)
The equation for the reaction is:
2HNO3(aq) + X2CO3(aq) → 2XNO3(aq)
+ H2O(l) + CO2(g)
[H = 1, C = 12, N =14, O = 16]
b) From your results and the information provided,
calculate the:
i)
Concentration of B in moldm-3
(4 marks)
ii)
Molar mass of B
(3 marks)
iii)
Value of X in X2CO3
(4 marks)
c) (i) Write two precautions taken in order to
obtain accurate readings from the pipette.
(2 marks)
(ii) What will be the colour of indicator in solution B and at the end point?
(2 marks)
Total = 23 marks
2. Candidates are advised to adhere strictly to the
instructions given. All observations and inferences must be recorded in the
table below. Identify any gas(es) given off.
You are provided with two samples X
and Y
|
|
Test |
Observation |
inference |
|
ai |
Put a little portion of sample X into a test tube
and heat. Test for gas(es) that may be evolved |
|
|
|
ii. |
Heat the portion strongly in (ai) above and allow to
cool. |
|
|
|
(b) |
Dissolve sample Y in about 5 cm3 of
distilled water and shake thoroughly. Divide the solution into two portions |
|
|
|
i. |
To the first portion, add dilute NaOH in drops |
|
|
|
ii. |
Then in excess |
|
|
|
iii. |
To the second portion, add dilute NH3
solution in drops |
|
|
|
iv. |
Then in excess |
|
|
Total = 19 marks
3. (a) i. What is the effect of adding NaCl crystals
on the boiling point of water?
(1mark)
ii) Mention one characteristics of boiling
(1 mark)
b) i.
Give one reason why the standard solution of NaOH cannot be prepared by weighing out
accurately a given mass of the solid salt and make it up to the required volume
of solution.
(1mark)
ii) Name one reagent that can be used in the laboratory for
( I) drying ammonia gas
(II) Testing for the presence of water
(1 mark)
(c) i. Name one
gas that cannot be prepared in open laboratory
(1mark)
ii. Give one use each of the following apparatus
in the laboratory.
(I)
Separating funnel
(II)
Gas jar
(1Mark)
Total = 8 marks
Answers
Volume of pipette used …25cm3
|
Titration |
Rough |
1 |
2 |
3 |
|
Final burette reading(cm3) |
13.50 |
13.30 |
13.20 |
13.30 |
|
Initial burette reading (cm3) |
0.00 |
0.00 |
0.00 |
0.00 |
|
Volume of A
used(cm3) |
13.50 |
13.20 |
13.20 |
13.30 |
(6 marks)
d) Calculate the average volume of A used.
13.30 +13.20 +13.30 = 39.8 cm3
39.8÷3 = 13.27 cm3
(2 marks)
The equation for the reaction is:
2HNO3(aq) + X2CO3(aq) → 2XNO3(aq)
+ H2O(l) + CO2(g)
[H = 1, C = 12, N =14, O = 16]
e) From your results and the information provided,
calculate the:
iv)
Concentration of B in moldm-3
Using the relation CAVA/CBVB
=a/b
CA = 0.15mol/dm VA
= 13.27cm,3 CB =
?? VB 25cm3
a. = 2 b. = 1
0.15 x 13.27/ CBX
25 = 2/1
1.9905/25CB = 2/1
Making CB
the subject formula 1.9905 ÷ 50 =
0.03981mol/dm
(4 marks)
v)
Molar mass of B
We know
that:
Mole = reacting mass ÷ molar mass
Mole = 0.03981mol/dm
Reacting mass = 4.2g/dm
So molar mass = 4.2 ÷ 0.03981 =105.5g/mol
(3 marks)
vi)
Value of X in X2CO3
From the molecular formula
X2CO3, note that
X is 2, C is 1 and O is 3
so
2X + C + 3O = 105.5g/mol
C= 12, O=16 then
2X + 12 +3(16) = 105.5
2X + 60 = 105.5
2X = 105.5 -60
2X = 45.5
X 45.5 ÷ 2
= 22.8
(4 marks)
f) (i) Write two precautions taken in order to
obtain accurate readings from the pipette.
·
I took the pipette reading at the lower meniscus
·
I ensure that pipette has no air bubbles
(2 marks)
(ii) What will be the colour of indicator in solution B and at the end point?
The indicator in solution B
is yellow and at end point is orange
(2 marks)
Total = 23 marks
4. Candidates are advised to adhere strictly to the
instructions given. All observations and inferences must be recorded in the
table below. Identify any gas(es) given off.
You are provided with two samples X
and Y
|
|
Test |
Observation |
inference |
|
ai |
Put a little portion of sample X into a test tube
and heat. Test for gas(es) that may be evolved |
Odourless, colourless gas, turns blue litmus red and
lime water milky |
Acidic gas CO2 present |
|
ii. |
Heat the portion strongly in (ai) above and allow to
cool. |
Yellow when heat and white when cool |
Zn2+ present |
|
(b) |
Dissolve sample Y in about 5 cm3 of
distilled water and shake thoroughly. Divide the solution into two portions |
Soluble to give a colourless solution |
Transition ions absent, soluble ions present |
|
i. |
To the first portion, add dilute NaOH in drops |
White precipitate |
Ca2+, Zn2+, Pb2+,
Al3+ likely present |
|
ii. |
Then in excess |
Soluble |
Al3+, Zn2+ and Pb2+ likely
present |
|
iii. |
To the second portion, add dilute NH3
solution in drops |
No visible reaction with dilute HCl, white
precipitate with NH3 solution |
Zn2+, Al3+ likely present |
|
iv. |
Then in excess |
White precipitate insoluble |
Al3+ confirmed |
Total = 19 marks
5. (a) i. What is the effect of adding NaCl crystals
on the boiling point of water?
Impurity increase the boiling point of substance so
the boiling point of water will increase.
(1mark)
ii) Mention one characteristics of boiling
·
It takes place at a fixed temperature for a given pressure.
·
It occurs throughout the entire volume of the liquid
(1 mark)
b) i.
Give one reason why the standard solution of NaOH cannot be prepared by weighing out
accurately a given mass of the solid salt and make it up to the required volume
of solution.
It is a deliquescent
substance
(1mark)
ii) Name one reagent that can be used in the laboratory for
( I) drying ammonia gas
Calcium oxide (CaO)
(II) Testing for the presence of water
Anhydrous copper (II)
tetraoxosulphate (VI) or Cobalt
chloride.
(1 mark)
(c) i. Name one
gas that cannot be prepared in open laboratory
Any of the following gases cannot
be prepared in open laboratory
Chlorine, hydrogen sulphide, carbon (II) oxide etc
(1mark)
ii. Give one use each of the following apparatus
in the laboratory.
(III)
Separating funnel
To separate two immiscible
liquid like water and oil
(IV)
Gas jar
For collection of gases
(1Mark)
Total = 8 marks
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