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SS2: CHEMISTRY - 2ND TERM

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  1. Water & Solution I | Week 1
    10 Topics
    |
    1 Quiz
  2. Water, Solution and Solubility | Week 2 & 3
    9 Topics
    |
    1 Quiz
  3. Air | Week 4
    4 Topics
    |
    1 Quiz
  4. Pollution | Week 5
    6 Topics
    |
    1 Quiz
  5. Rate of Chemical Reaction | Week 6 & 7
    6 Topics
    |
    1 Quiz
  6. Energy and Energy Effect I | Week 8 & 9
    7 Topics
    |
    1 Quiz
  7. Energy and Energy Effect II | Week 10 & 11
    6 Topics
    |
    1 Quiz
  8. Chemical Equilibrium | Week 12
    8 Topics
    |
    1 Quiz

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SS2: CHEMISTRY – 2ND TERM Air | Week 4 SS2: Chemistry Theory Questions – Air
Lesson 3, Topic 4
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SS2: Chemistry Theory Questions – Air

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Question 1:

[a]  give the constituents of atmospheric air and indicate its approximate percentage composition.
[b] Give three reasons why air is classified as a mixture.
[c] How would you demonstrate that rainwater contains dissolved air?

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Question 2:

[a] Calculate the volume of air required to produce 35.0g of phosphorus(V) oxide during the combustion of an excess of phosphorus [O= 16, P = 31]. All gas volumes measured at the same temperature and pressure.
[b] Name a solution that can be used to detect the presence of oxygen in the atmosphere.

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Question 1

[a]  give the constituents of atmospheric air and indicate its approximate percentage composition.

Answer:

Nitrogen – 78%
Oxygen – 21%
CO2 – 0.03%
Water vapour – Variable
Noble gases – 1%
Impurities (H2S, SO4) – Variable

[b] Give three reasons why air is classified as a mixture.

Answer:

– The constituents can be easily separated by physical methods
– The constituents of air still retain their individual properties
– The composition of air cannot be represented by a chemical formula like ‘Compound’.

[c] How would you demonstrate that rainwater contains dissolved air?

Answer:

By carrying out an experiment of burning phosphorous in air, using the collected rainwater. After burning out the oxygen, read the volume of the mixture of gases left.

Question 2

[a] Calculate the volume of air required to produce 35.0g of phosphorus(V) oxide during the combustion of an excess of phosphorus [O= 16, P = 31]. All gas volumes measured at the same temperature and pressure.

Solution:

Equation of reaction

4P + 5O2  ⇒ 2P2O5

Using first principle.

284g of P2O5 ⇒ 5 x 22.4dm3

284g of P2O5 ⇒ 112dm3

35g of P2O5⇒ \( \frac{112}{284} \scriptsize \: \times \: 35  \\ \scriptsize = 13.80dm^3\)

Since oxygen contains 20cm3 by 100cm3 of air

1cm3 will contain \( \frac{100}{20} \scriptsize = 5cm^3 \)

13800cm3 will contain 13800 x 5 = 6900cm3

[b] Name a solution that can be used to detect the presence of oxygen in the atmosphere.

Answer: Alkaline pyrogallol – Pyrogallol is an organic compound with the formula C6H3(OH)3. It is a white, water-soluble solid although samples are typically brownish because of its sensitivity toward oxygen.

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