Q16 / 160 · TS EAMCET 2009 · Chemistry

Aluminium reacts with $\mathrm{NaOH}$ and forms compound ' $X$ '. If the coordination number of aluminium in ' $X$ ' is 6 , the correct formula of $X$ is

  • A
    $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_4(\mathrm{OH})_2\right]^{+}$
  • B
    $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_3(\mathrm{OH})_3\right]$
  • Correct answer
    C
    $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_2(\mathrm{OH})_4\right]^{-}$
  • D
    $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_6\right](\mathrm{OH})_3$

Solution

$2 \mathrm{Al}+2 \mathrm{NaOH}+2 \mathrm{H}_2 \mathrm{O} \longrightarrow \underset{\substack{\text { sodium } \\ \text { meta } \\ \text { aluminate }}}{2 \mathrm{NaAlO}_2}+3 \mathrm{H}_2$ Sodium metaaluminate, thus formed, is soluble in water and changes into the complex $\left[\mathrm{Al}\left(\mathrm{H}_2 \mathrm{O}\right)_2(\mathrm{OH})_4\right]^{-}$, in which coordination number of $\mathrm{Al}$ is 6 .

Hence, the correct answer is option: 'C'.

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