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A certain salt `(X)` gives the following tests : (a) Its aqueous solution is alkaline to litmus. (b) On strong heating. It sweels to give a glassy bead. ( c) When conc `H_(2) SO_(4)` is added to a hot concentrated solution of `(X)`, white crystals of a weak acid separates out. Identify `(X)` and write down the chemical equations for reaction at steps `a, b` and `c`. |
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Answer» a. Since the aqueous solution of salt `(X)` is alkaline to litmus, it must be a salt of a strong base and a weak acid. b. Since `(X)` swells up to a glassy material `(Y)` on strong heating , therefore,`(X)` must be borax and `(Y)` must be a mixture of sodium metaborate and boric anhydride. c. On addition of conc. `H_(2)SO_(4)` to hot solution of `(X)`, white crystals of an acid `(Z)` serarates out, therefore, `(Z)` must be orthoboric acid. Various reactions involved in steps (a),(b) and (c) are as follows: `underset(("X")"Borax")(Na_(2)B_(4)O_(7)).10H_(2)O+H_(2)Otounderset("Strong base")(2NaOH)+underset("Weak acid")(H_(2)B_(4)O_(7))+8 H_(2)O` `underset(("X"))(Na_(2)B_(4)O_(7)).10H_(2)Ooverset(Delta)toNa_(2)B_(4)O_(7)+10H_(2)O` `Na_(2)B_(4)O_(7)overset(Delta)tounderset("Glassy material(Y)")ubrace(2NaBO_(2)+B_(2)O_(3))` `Na_(2)B_(4)O_(7).10H_(2)O+underset(Conc.)(H_(2)SO_(4))tounderset("Boric acid(Z)")(4H_(3)BO_(3))+Na_(2)SO_(4)+5H_(2)O` Hence, `(X)` is borax,`Na_(2)B_(4)O_(7).10H_(2)O` `(Y)` is mixture of sodium metaborate, `NaBO_(2)`,and boric anhydride, `B_(2)O_(3)`. `(Z)` is boric acid, `B(OH)_(3)`. |
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