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Preparations and Uses of Caustic Soda

When 20.02 ...

Question

When $20.02$ g of white solid (X) was heated, $4.4$ g of acid gas (A) that turned lime water milky was driven off together with $1.8$ g if a gas (B) which condenses to a colourless liquid. The solid (Y) that remained is dissolved in water to give an alkaline solution, with which excess barium chloride solution gave a white precipitate (Z). The precipitate effervesced with acid giving off carbon dioxide. Compound (A), (B) and (Y) are identified as:
(A) : $C O_{2}$ , (B) : $H_{2} O$ and (Y) : $K_{2} C O_{3}$
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Solution

The compound (X) is metal bicarbonate. On heating, it decomposes to form metal carbonate, carbon dioxide and water.
$2 M H C O_{3} 2 (m + 61) g ⟶ M_{2} C O_{3} + H_{2} O + C O_{2} 44 g$
Let M g/mol be tha molar mass of the metal. The molar mass of the metal carbonate will be M+61 g/mol. The molar mass of carbon dioxide is 44 g/mol.
2 moles of metal carbonate gives 1 mole of carbon dioxide.
$\frac{2 (M + 61)}{20.02} = \frac{44}{4.4}$
Thus, M = 39.1.
Hence, the metal is potassium.
Thus, (X) is $K H C O_{3}$ , (Y) is $K_{2} C O_{3}$ , (Z) is $B a C O_{3}$ , (A) is $C O_{2}$ and (B) is $H_{2} O$ .
When potassium carbonate is dissolved in water to form alkaline solution, which is then treated with excess barium chloride solution a white ppt of barium carbonate is obtained which is compound (Z).
$K_{2} C O_{3} + 2 H_{2} O \to 2 K O H + H_{2} O + C O_{2}$
$C O_{2} + B a C l_{2} + H_{2} O \to B a C O_{3} + 2 H C l$
Barium mcarbonate on treatment with acid forms carbon dioxide gas.
$b a C O_{3} + 2 H C l \to B a C l_{2} + H_{2} O + C O_{2} ↑$

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X

was heated

4.4 g

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that turned lime water milky was driven of together with

1.8 g

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(B)

which condensed to a colourless liquid the solid that remained

(Y)

dissolved in water to give an alkaline solution, which with excess of

B a C l_{2}

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(i) A white amorphous powder (A) on strongly heating gives a colourless non-combustile gas (B) and solid (C).
(ii) The gas (B) turns lime water milky and turbidity disappear with the passage of excess of gas.
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K_{4} [F e (C N)_{6}]

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Z n C O_{3}

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Q. A white amorphous powder (A) when heated gives a colourless gas (B), which turns lime water milky and the residue (C) which is yellow when hot but white when cold. The residue (C) dissolves in dilute HCl and the resulting solution gives a white precipitate on addition of potassium ferrocyanide solution. (A) dissolves in dilute HCl with the evolution of a gas which is identical in all respects with (B). The solution of (A) as obtained above gives a white precipitate (D) on addition of excess of

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Z n C O_{3}

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C O_{2}

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Z n O

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Z n S

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Z n (O H)_{2}

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Q. A white amorphous powder

A

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B

and a solid

C

. The latter compound assumes a yellow color on heating and changes to white on cooling.

C

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H C l

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(A) :

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P b C l_{2}

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