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What is Air?
Air is a mixture of gases - nitrogen, oxygen, carbon(IV) oxide, water vapor and noble gases. The following observations confirm air to be a mixture:
- the composition of air is not quite constant. Variations in composition have been found when samples of air are taken from different parts of the earth. This implies that if air were a compound, its composition would be definite or constant.
- if air is dissolved in water and boiled out again, it will be observed that the percentage of oxygen in the air is increased from 21% to about 30%. The increase in percentage of oxygen only shows that water usually contain dissolved oxygen, even more than nitrogen (oxygen is about twice more soluble in water than nitrogen). The dissolution and release of air from water is a physical process which implies that air is a mixture.
- when liquid air is heated, nitrogen evaporates earlier, leaving almost pure oxygen. This implies that components of air are easily separable by physical methods.
- a mixture of carbon(IV) oxide, nitrogen, oxygen, water vapor and noble gases in appropriate ratio does not produce any observable change identifiable with chemical reactions (such as evolution of heat, explosion and volume change), but the mixture is similar to ordinary air in everyway.
- the composition of air is not represented by any simple chemical formula, unlike if it were a compound.
Composition of Air
The constituents of air and their percentage composition are given below:
Nitrogen - 78.1% - 4/5 of volume of air
Oxygen - 20.9% -1/5 of volume of air.
Carbon dioxide - 0.03%.
Water vapor - variable.
Noble gases - about 1%
Impurities (example, H2S, SO2, e.t.c.) - variable.
The above statistics shows that nitrogen and oxygen are the two main gases of the air, occupying about 4/5 and
1/5 by volume respectively.
How to Determine the Molar Mass of Air
To Determine the Presence and Proportion of The Constituents of Air
The presence and proportion of oxygen in air can be determined by burning certain metals, example, copper, lead and magnesium in air. The oxygen of the air combines with these metals to form oxides, which are greater in masses than the pure metals.
The difference in mass is the oxygen present in the volume of air used - this procedure can be employed to estimate the volume of oxygen in the air. The equations for the chemical reactions are:
2Cu(s) + O2(g) → 2CuO(s) Copper(II) oxide
2Pb(s) + O2(g) → 2PbO(s) Lead(II) oxide
2Mg(s) + O2(g) → 2MgO(s)
Phosphorus can also be burnt in a measured
volume of air to obtain by volume the proportion of oxygen in air. The equation of the reaction is:
P4(s) + 5O2(g) → P4O10(s) Phosphorus(V) oxide
For convenience, white phosphorus is used. White phosphorus catches fire very easily (for this reason, it is stored under water).
Note: only the oxygen component of air supports combustion, others, i.e., CO2, N2, and water do not.
To obtain a more accurate determination of the proportion of oxygen by volume in air, we can use the smoldering of phosphorus in air, or by passing air into alkaline pyrogallol, or into benzene-1,2,3 - triol, which absorbs its oxygen.
When white phosphorus is exposed to a measured volume of air, it smolders as it absorbs oxygen from the air. The volume of the absorbed oxygen is measured, and the percentage composition is calculated to be about 20.8%. The chemical change that occurs is same with that of the combustion of phosphorus in air:
P4(s) + 5O2(g) → P4O10(s)
When a measured volume of air is passed into alkaline pyrogallol or benzene-1,2,3-triol, only the oxygen component is absorbed. The volume of the absorbed oxygen is measured and its percentage composition can also be determined.
The occurrence of carbon(IV) oxide in air is traceable to the combustion of fuels, e.g. coal, wood, petrol and paraffin - these materials are composed mainly of carbon.
C(s) + O2(g) → CO2(g)
It is also present in the air through the process of respiration – all animals and plants produce CO2 as a by-product of respiration, which is released into the atmosphere. The decay of organic material also releases CO2 into the atmosphere. For the fact that plants require CO2 to synthesis carbohydrates, and also for the fact that CO2 dissolves in the water of the oceans, the percentage of CO2 in air remains constant at 0.03% by volume, in spite of the enormous amount produced into the atmosphere.
The presence and proportion of CO2 in the air can be determined by passing a measured volume of air into a solution of calcium hydroxide (also called lime water). Calcium hydroxide solution absorbs CO2 in limited amount to give white precipitate of CaCO3, and in excess amount to give a milky appearance.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)
Calcium hydrogen trioxocarbonate(IV).
The milky appearance is due to calcium hydrogen trioxocarbonate(IV), Ca(HCO3)2 produced.
The volume of CO2 absorbed is measured, and its percentage composition calculated. Other substances that can be used to absorb CO2 are concentrated solutions of KOH and NaOH (these will produce soluble carbonates with limited CO2 ; and hydrogentrioxocarbonate(IV) with excess CO2.
Solid NaOH can also be used. Solid NaOH absorbs water from the air to form a solution, which then absorbs CO2 to form sodium trioxocarbonate(IV) decahydrate. The decahydrate loses 9 of its water of crystallization, absorbs more CO2 and forms sodium hydrogen trioxocarbonate(IV).
2NaOH(s) + 9H2O(l) + CO2(g) → Na2CO3.10H2O(s)
Na2CO3 .H2O(s) + CO2(g) → 2NaHCO3(s)
The evaporation of water from oceans, rivers, lakes etc, produces the water vapor of air. Its presence and proportion in the air can be found by passing a measured volume of air through some substances which absorb water, such as anhydrous calcium chloride and conc. tetraoxosulphate(VI) acid.
In a day or two, a solution of the compound will be obtained, while the volume of air decreases. The volume of water vapor thereby absorbed is measured, and its percentage composition calculated - the results vary from place to place.
Nitrogen is almost inert; therefore, there is no suitable chemical procedure to test it in the
presence of the other components.
Hence, the other components are usually removed from the air, leaving behind nitrogen for complex test procedures. The following is a procedure to separate nitrogen from air:
A given volume of air is passed through a deliquescent substance to remove water
vapor, after which, it is passed into a solution of slaked lime, i.e. calcium hydroxide, where the CO2 component is absorbed.
It is moved onto a furnace where the oxygen component burns copper to give copper(II) oxide. The gas left after this process is mainly nitrogen, which is not removed by any known chemical method.
Note: * The presence of the noble gases in atmospheric nitrogen makes it denser than pure nitrogen obtained from its compounds. In the industry, either nitrogen or oxygen is obtained from liquid air (containing mainly oxygen and nitrogen) by fractional distillation. Nitrogen boils at 77 K, argon which is the major noble gas in the air boils at 87 K, while oxygen boils at 90 K.
* By fractional distillation, pure nitrogen is obtained.
Air Impurities or PollutantsAir, especially in the industrial areas contains certain particles, which
pollute it. These include hydrogen sulphide (H2S), sulphur(IV) oxide (SO2), the
oxides of nitrogen, carbon monoxide (CO), dust and other solid particles such as
lead. An evidence of these pollutants in the air is the tarnishing of silver-
this is due to the presence of H2S, which forms a black layer of silver sulphide
on the sliver.
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