11th Class Chemistry The p-Block Elements-I

  • question_answer 38) A certain salt (X) gives the following results: (i) Its solution is alkaline to litmus. (ii) It swells upto glassy material \[(Y)\] on strong heating. (iii) When concentrated \[{{H}_{2}}S{{O}_{4}}\] is added to hot solution of (X), a white crystal of an acid (Z) separates out. Write equations for all the above reactions and identify (X), (Y) and (Z).

    Answer:

    The salt (X) dissolves and its aqueous solution is alkaline to litmus, it must be a salt of a strong base and a weak acid. On heating it forms a glassy material, hence (X) is a borax. \[N{{a}_{2}}{{B}_{4}}{{O}_{7}}+7{{H}_{2}}O\xrightarrow{{}}\underset{Strong\,alkali}{\mathop{2NaOH}}\,+4{{H}_{3}}B{{O}_{3}}\] \[\underset{(X)}{\mathop{N{{a}_{2}}{{B}_{4}}{{O}_{7}}}}\,\cdot 10{{H}_{2}}O\xrightarrow{Heat}N{{a}_{2}}{{B}_{4}}{{O}_{7}}+10{{H}_{2}}O\] \[N{{a}_{2}}{{B}_{4}}{{O}_{7}}\xrightarrow{Heat}2NaB{{O}_{2}}+{{B}_{2}}{{O}_{3}}\] \[N{{a}_{2}}{{B}_{4}}{{O}_{7}}+{{H}_{2}}S{{O}_{4}}\xrightarrow{{}}\underbrace{N{{a}_{2}}S{{O}_{4}}+{{H}_{2}}{{B}_{4}}{{O}_{7}}}_{\underset{(Y)}{\mathop{Glass\,material}}\,}\] \[{{H}_{2}}{{B}_{4}}{{O}_{7}}+5{{H}_{2}}O\xrightarrow{{}}\underset{\underset{\begin{smallmatrix} Orthoboric\,acid \\ (White\,crystals) \end{smallmatrix}}{\mathop{(Z)}}\,}{\mathop{4{{H}_{3}}N{{O}_{3}}}}\,\]  


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