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  • A certain salt X, gives the following results. (i) Its aqueous solution is alkaline to litmus. (ii) It swells up to a glassy material Y on strong heating. (iii) When conc. H_2SO_4 is added to a hot solution of X,white crystal of an acid Z separates out.

11.30     A certain salt X, gives the following results.
              (i) Its aqueous solution is alkaline to litmus.
              (ii) It swells up to a glassy material Y on strong heating.
              (iii) When conc. H_{2}SO_{4} is added to a hot solution of X,white crystal of an acid Z separates out.
              Write equations for all the above reactions and identify X, Y and Z.

Answers (1)

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As per the given information, the compound X is a salt of a strong base and a weak acid because salt is alkaline to litmus. And when X is heated it swells to become Y. Thus, this must be borax because borax on heating, loses water molecule and swells to form sodium metaborate if we continue to heat it becomes glassy material Y. Hence Y should be a mixture of two compound boric anhydrides and metaborate.

The reactions are shown here-
Na_{2}B_{4}O_{7}+7H_{2}O\rightarrow 2NaOH+4H_{3}BO_{3} (orthoboric acid)

Na_{2}B_{4}O_{7}.10H_{2}O\overset{\Delta }{\rightarrow} Na_{2}B_{4}O_7\overset{\Delta }{\rightarrow}B_{2}O_{3}+2NaBO_{2}
  (Borax X)                  (sodium metaborate)  (boric anhydride)
                                              |_______Y_________|

When a concentrated acid is added to borax, white crystals are formed 

Na_{2}B_{4}O_{7}.10H_{2}O+H_{2}SO_{4}\overset{\Delta }{\rightarrow} Na_{2}SO_{4}+4H_{3}BO_{3}+5H_{2}O
                                                                                          (orthoboric acid Z)     

Posted by

manish

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