I look at this later...

Q18

Wet powdered calcium carbonate serves to allow the carbonate to react with some acids, or at least acidic gases. The "wet" condition does not really dissolve the calcium carbonate at all since calcium carbonate is poorly soluble in water; instead, it helps to dissolve the acidic gases into aqueous acids which then react with the calcium carbonate. The acid-carbonate reaction will only proceed if at least one of them is aqueous (two solids won't react).

So, the "wet" condition makes the reaction of calcium carbonate with acidic gases possible. We are basically looking for non-acidic gases here since the question wants the gases which are NOT removed by the wet calcium carbonate.

The non-acidic gases in the list are
- carbon monoxide
- nitrogen monoxide

The other four are acidic gases capable of turning into acids. You would have probably known sulfur dioxide, carbon dioxide and nitrogen dioxide. Phosphorus (V) oxide, however, is not quite covered in the syllabus. For your information, however, phosphoric acid is a phosphorus-based acid which is actually more acidic than sulfuric acid when they are used in equal concentrations. Phosphorus oxide is an acidic oxide which dissolves in water to form phosphoric acid.

So, we have two gases which are not removed by the calcium carbonate.

Q19

The role of a catalytic converter is to speed up some reactions in the exhaust system so that more harmful products like unburnt hydrocarbons, nitrogen monoxide and carbon monoxide are converted into less harmful products like nitrogen, nitrogen dioxide and carbon dioxide.

(A) (D)

When unburnt hydrocarbons undergo complete combustion, the main resulting products formed are carbon dioxide and water, so the percentage of water vapour increases. Less carbon monoxide will be formed as the carbon monoxide produced also undergo oxidation to form carbon dioxide in the complete combustion, thus reducing the percentage of carbon monoxide remaining.

(A) True statement.
(D) True statement,

(B) (C)

Nitrogen monoxide has a formula NO. Upon inspection, since O has an oxidation number of -2, N must have an oxidation number of +2 (have you learnt the topic on redox reaction?) to keep the overall oxidation number of the entire compound zero.

Nitrogen gas itself has a formula N2, and the N here has an oxidation number of zero since the entire molecule, which is only made up of a single element, must have an overall oxidation number of zero.

In the change of the oxidation number of nitrogen from -2 in NO to 0 in N2, the value has actually increased, so N is being oxidised and therefore this represents an oxidation reaction.

(B) True statement.
(C) False statement.

i dont really understand the entire qn19 as i havent learn oxidation, reduction, combustion, redox reaction, thermal decomposition etc

This topic requires you to know the chapters on air and redox.

For the air component, you must realise that catalytic converters allow a more complete reaction to occur so as to reduce the presence of the harmful carbon monoxide and nitrogen monoxide by the following reactions:

nitrogen monoxide + carbon monoxide ==> nitrogen gas + carbon dioxide

nitrogen monoxide + oxygen ==> nitrogen dioxide (but nitrogen dioxide is also bad actually)

carbon monoxide + oxygen ==> carbon dioxide (less evil than carbon monoxide)

For the redox component, you must understand the principles behind oxidation and reduction.

- oxidation is achieved by a gain of oxygen, a loss of hydrogen, a loss of electrons or an increase in oxidation number/state

- reduction is achieved by a loss of oxygen, a gain in hydrogen, a gain in electrons or a decrease in oxidation number/state

Once you have read up and practised sufficiently on the oxidation number/state section, this question should be a breeze for you.

ok will go read up on it and see if i am able to understand the topic :)