P-Block Elements Ques 219
Question: A certain salt (X) gives the following tests:
(i) Its aqueous solution is alkaline to litmus.
(ii) On strongly heating it swells to give a glassy bead
(iii) When concentrated sulphuric acid is added to hot concentrated solution of (x), crystals of $ H_3BO_3 $ separate out. Identify the colour of these crystals.
Options:
A) Violet
B) Blue
C) Brown
D) White
Show Answer
Answer:
Correct Answer: D
Solution:
(X) is borax, $ Na_2B_4O_7,10H_2O $ (i) $ Na_2B_4O_7+7H_2O\rightarrow \underset{\begin{matrix} (strong) \\ Base \\ \end{matrix}}{\mathop{2NaOH}}+\underset{\begin{matrix} (weak) \\ acid \\ \end{matrix}}{\mathop{4H_3BO_3}} $
Due to presence of $ NaOH $ , the aqueous solution is alkaline to litmus.
(ii) $ Na_2B_4O_7,10H_2O\xrightarrow[(swells)]{Heat}Na_2B_4O_7+10H_2O $ $ \to 2NaBO_2+\underset{Glassybead}{\mathop{B_2O_3}} $
(iii) $ Na_2B_4O_7+H_2SO_4+5H_2O\to Na_2SO_4+\underset{whitecrystals}{\mathop{4H_3BO_3}} $
