JEE Main & Advanced Sample Paper JEE Main Sample Paper-12

  • question_answer
    Unknown sample [A] on heating swells up first and aqueous solution of [A] 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 :

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


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