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Question 15 Marks
How will you prove that Hydrochloric acid contains (i) Hydrogen (ii) Chlorine?
Write equations for the reactions. 
Answer
We can prove that hydrochloric acid contains both hydrogen and chlorine by the following experiment.
Take a voltameter used for electrolysis of water, fitted with the platinum cathode and a graphite anode.
Into the voltameter pour 4 molar $HCl$ and pass direct current.
It is seen that a colourless gas is evolved at the cathode and a greenish gas is evolved at the anode. When a burning splinter is brought near a colorless gas, it bursts into a flame thereby proving that it is hydrogen gas.
When moist starch iodide paper is held in the greenish-yellow gas, it turns blue-black, thereby proving that the gas is chlorine.
$
2 HCl \rightarrow H _2+ Cl _2
$
This experiment proves that hydrochloric acid contains both hydrogen and chlorine.
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Question 25 Marks
Draw a labelled diagram for the laboratory preparation of hydrogen chloride gas and answer the following.
$a.$ Name the acid used. Why is this particular acid preferred to other acids?
$b.$ Give the balanced equation for the reaction.
$c.$ Name the drying agent used in drying hydrogen chloride gas.
$d.$ Phosphorous pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas. Why?
$e.$ Why is the direct absorption of $\text{HCl}$ gas in water not feasible?
$f.$ What arrangement is done to dissolve $\text{HCl}$ gas in the water?
Answer
Labelled Diagram for laboratory preparation of Hydrogen chloride is :

$a.$ Concentrated $H_2SO_4$
$b.$ The balanced equation for the reaction: $\ce{NaCl + H2SO4 ->[<200^\circ C]NaHSO4 + HCl}$
$c.$ The drying agent used in drying hydrogen chloride gas is conc. sulphuric acid.d. Phosphorous pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas because they react with hydrogen chloride
$\ce{2P2O5 + 3HCl -> POCl3 + 3HPO3}$
$\ce{CaO + 2HCl -> CaCl2 + H2O}$
$e.$ Hydrogen chloride gas is highly soluble in water. Therefore, it is not collected over water.
$f.$ The funnel arrangement is done to dissolve $\text{HCl}$ gas in water.
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[5 Mark Question Answer] - Chemistry STD 10 Questions - Vidyadip