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  • A violet colour flame was observed when a white crystalline compound was heated. It also swells up. Following reactions are given by the said compound: 1. With <img alt="\mathrm{\mathrm{B

A violet colour flame was observed when a white crystalline compound was heated. It also swells up. Following reactions are given by the said compound:

1. With \mathrm{\mathrm{BaCl}_2}, in presence of \mathrm{\mathrm{HCl}}, it gives a white precipitate.

2. White gelatinous precipitate is formed on reaction with excess of ammonium hydroxide which dissolves in sodium hydroxide but appears back when it is boiled with concentrated solution of \mathrm{NH_4Cl}

3.On treatment with concentrated solution, it gives a yellow coloured precipitate. Identify the compound.

Option: 1

\mathrm{\mathrm{K}_2 \mathrm{SO}_4 \cdot \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3, 24 \mathrm{H}_2 \mathrm{O}}


Option: 2

\mathrm{K}_2 \mathrm{SO}_4


Option: 3

\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3


Option: 4

\mathrm{NaOH}


Answers (1)

best_answer

Potash Alum \mathrm{\left(\mathrm{K}_2 \mathrm{SO}_4 \cdot \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3 \cdot 24 \mathrm{H}_2 \mathrm{O}\right)} is the given compound, it swells on hating due to evolution of water molecules.

\mathrm{ \mathrm{K}_2 \mathrm{SO}_4 \cdot \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3 \cdot 24 \mathrm{H}_2 \mathrm{O} \underset{\text { swells }}{\stackrel{\Delta }{\longrightarrow}} \mathrm{K}_2 \mathrm{SO}_4 \cdot \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3+24 \mathrm{H}_2 \mathrm{O} }

Violet-coloured flame is due to the presence of potassium salt.

1. sulphate ions prevention solution combines with \mathrm{Ba}^{2+} ion to give white ppt.
\mathrm{ \mathrm{Ba}^{2+}+\mathrm{SO}_4{ }^{2-} \rightarrow \mathrm{BaSO}_4 \text {. } }

2. \mathrm{ \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3+6 \mathrm{NH}_4 \mathrm{OH} \rightarrow 2 \mathrm{Al}(\mathrm{OH})_3+3\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 \downarrow \\ }

\mathrm{\mathrm{Al}(\mathrm{OH})_3+\mathrm{NaOH} \rightarrow \underset{\textup{soluble}}{\mathrm{NaAlO}_2}+2 \mathrm{H}_2 \mathrm{O} \\ }\mathrm{ \mathrm{NaAlO}_2+\mathrm{NH}_4 \mathrm{Cl}+\mathrm{H}_2 \mathrm{O} \stackrel{\triangle}{\longrightarrow} \mathrm{Al}(\mathrm{OH})_3+\mathrm{NaCl}+\mathrm{NH}_3 }

3. \mathrm{ 2 \mathrm{~K}_2 \mathrm{SO}_4+\mathrm{Na}_4\left[\mathrm{CO}\left(\mathrm{NO}_2\right)_6\right] \rightarrow \underset{\textup{Yellow ppt.}}{\mathrm{K}_4\left[\mathrm{CO}\left(\mathrm{NO}_2\right)_6\right]}+2 \mathrm{Na}_2 \mathrm{SO}_4 }

Hence option 1 is correct.

Posted by

Ritika Harsh

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