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the question is gas M is formed as a by-product during the manufacture of NaOH. The gas M reacts with
dry slaked lime to give a compound N which is used to make drinking water free from
germs. Identify M and N giving chemical equations of the reactions involved. marks (3)

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Sodium chloride is used for manufacture of sodium hydroxide which is called chloralkali process. 
In this process, chlorine and hydrogen gas are formed as byproducts along with sodium hydroxide. 
Chlorine gas gives bleaching power when reacts with lime water and used as bleaching agent in chemical industries. 
2NaCl(aq)+2H2O(l)→ 2NaOH(aq)+Cl2(g)+H2(g) Ca(OH)2+Cl2⟶ CaOCl2+H2O 
Therefore, the gas M is chlorine (Cl2).
Compound N is calcium oxychloride (CaOCl2) which is commonly known as bleaching powder and used to make drinking water free from germs .
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The gas formed during the preparation of NaOHNaOHis Cl2Cl2 gas, thus M=Cl2Cl2.
Slaked Lime is Ca(OH)2Ca(OH)2, 
                     thus when Cl2Cl2 reacts to slaked like then CaOCl2CaOCl2 i.e. Calcium Oxy Chloride or Bleaching Powder.
Thus N=CaOCl2
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In the manufacture of sodium hydroxide, hydrogen gas and chlorine gas (M) are formed as by-products. When chlorine gas (M) reacts with lime water, it forms calcium oxychloride (bleaching powder) N. The reactions are 2NaCI(aq) + 2H2O(l) → 2NaOH(aq) + Cl2(g) + H2(g)

 M → Cl2 (Chlorine gas) Ca(OH)2 (s) + Cl2 (g) →  CaOCl2 (s) + H2O N — Calcium oxychloride (bleaching powder) 

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Industrial production of sodium hydroxide-
Ca(OH)2 (aq) + Na2CO3 →→ CaCO3↓↓ + 2NaOH(aq)

The gas M formed is carbon dioxide (CO2) as a N product which is Calcium carbonate (CaCO3) in above reaction.

Carbon dioxide reacts with limewater (Ca(OH)2) to give calcium carbonate, which is used as bleaching agent in chemical industries. The reaction is as follows:
CO2 + Ca(OH)2 →→ CaCO3 + H2O
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