Form 1:
Air and Combustion

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To find the composition of active part of air using alkaline pyrogallol Procedure

Measure about 2cm3 of dilute sodium hydroxide into a graduated gas jar.

Record the volume of the graduated cylinder V1.

Place about two spatula end full of pyrogallol/1, 2, 3-trihydroxobenzene into the gas jar.

Immediately place a cover slip firmly on the mouth of the gas jar.

Swirl thoroughly for about two minutes.

Invert the gas jar in a trough/basin containing water.

Measure the volume of air in the gas jar V2

Sample observations

Colour of pyrogallol/1, 2, 3-trihydroxobenzene changes to brown.

Level of water in gas jar rises when inverted in basin/trough.

Volume of gas jar /air in gas jar V1= 800cm3

Volume of gas jar /air in gas jar after shaking with alkaline pyrogallol/1, 2, 3- trihydroxobenzene V2= 640 cm3

Sample questions

1. Which gas is absorbed by alkaline pyrogallol/1,2,3-trihydroxobenzene

Oxygen

2. Calculate the

(i) % of active part of air

V1-V2/V1 x 100% => (800cm3 - 640 cm3)/ 800cm3x 100% = 20%

(ii) % of inactive part of air

V2/V1 x 100% => 640 cm3/800cm3 x 100% = 80%

To establish the presence of carbon (IV) oxide in air using lime water

Pass tap water slowly into an empty flask as in the set up below

Sample observation questions

1. What is the purpose of paper cover?

To ensure no air enters into the lime water.

2. What happens when water enters the flask?

It forces the air from the flask into the lime water.

3. What is observed when the air is bubbled in the lime water?

A white precipitate is formed. The white precipitate dissolves on prolonged bubbling of air.

4. (a) Identify the compound that form:

(i)lime water
Calcium hydroxide / Ca(OH)2

(ii) White precipitate
Calcium carbonate/ CaCO3

(iii) When the white precipitate dissolves
Calcium hydrogen carbonate/ CaHCO3

(b)Write the chemical equation for the reaction that take place when:

(i) White precipitate is formed

Calcium hydroxide + carbon (IV) oxide -> Calcium carbonate + water
Ca (OH) 2(aq) + CO2 (g) -> CaCO3(s) + H2O (l)

(ii) White precipitate dissolves

Calcium carbonate + water+ carbon (IV) oxide -> Calcium hydrogen carbonate
CaCO3(s) + H2O (l) + CO2 (g) -> CaHCO3 (aq)

5. State the chemical test for the presence of carbon (IV) oxide gas based on 4(a) and (b) above:

Carbon (IV) oxide forms a white precipitate with lime water that dissolves in excess of the gas.

6. State the composition of carbon (IV) oxide gas by volume in the air.

About 0.03% by volume

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