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# An aqueous solution of a crystalline salt reacts with dilute HCl to give a yello...

### Question

An aqueous solution of a crystalline salt reacts with dilute HCl to give a yellow precipitate and a gas that turned dichromate paper green. The crystalline salt may be

### Options

A) $$Na_2S_2O_3 \cdot 5H_2O$$

B) $$Na_2CO_3 \cdot 10H_2O$$

C) $$Na_2S$$

D) $$NaHCO_3$$

## Dicussion (1)

• The Correct Answer is A.

Firstly, the two gases that turn dichromate paper green are H2S and SO2. Also, we know that what we want is a crystalline salt and we know that every crystalline salt usually have at least a mole Water of Crystalization attached to the Salt, that leaves us with just A and B.

Out of the two only $$Na_{2}S_{2}O_{3}.5H_{2}O$$ contains Sulphur and thus is able to give out SO2 and deposit of Sulphur (Yellow Deposit) on reacting with HCl. $$Na_{2}S$$ on the other hand will give out H2S but no yellow deposit of Sulphur and this is not what we want.

We can therefore reach a logical conclusion that the CORRECT ANSWER is A, $$Na_{2}S_{2}O_{3}.5H_{2}O$$.