JEE Main & Advanced Sample Paper JEE Main Sample Paper-37

  • question_answer
    Unknown sample  on heating swells up first and aqueous solution of  gives white ppt. with \[NaOH\] which becomes soluble with excess of \[NaOH\] addition -

    A)  Borax                          

    B)  Alum

    C)  Microcosmic salt           

    D)  None

    Correct Answer: B

    Solution :

     \[{{K}_{2}}S{{O}_{4}}.A{{l}_{2}}{{(S{{O}_{4}})}_{3}}.24{{H}_{2}}O\xrightarrow[swells]{\Delta }\underbrace{{{K}_{2}}S{{O}_{4}}+A{{l}_{2}}{{(S{{O}_{4}})}_{3}}}_{Amorphous\,powder}+24{{H}_{2}}O\] \[{{K}_{2}}S{{O}_{4}}.A{{l}_{2}}{{(S{{O}_{4}})}_{3}}.24{{H}_{2}}O+8NaOH\xrightarrow{{}}\underset{\text{soluble}}{\mathop{2KOH}}\,+\underset{(white\,ppt.)}{\mathop{2Al{{(OH)}_{3}}}}\,\]\[\downarrow \underset{\text{soluble}}{\mathop{+4N{{a}_{2}}S{{O}_{4}}}}\,\xrightarrow{\begin{smallmatrix}  Excess\,\,of \\  NaOH \end{smallmatrix}}\underset{\text{soluble}}{\mathop{{{[Al{{(OH)}_{4}}]}^{-}}}}\,\]


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