The P Block Elements - Result Question 67
####70. In which of the following arrangements the given sequence is not strictly according to the property indicated against it?
[2012 M]
(a) $HF<HCl<HBr<HI$ : increasing acidic strength
(b) $H_2 O<H_2 S<H_2 Se<H_2 Te$ : increasing $p K_a$ values
(c) $NH_3<PH_3<AsH_3<SbH_3$ : increasing acidic character
(d) $CO_2<SiO_2<SnO_2<PbO_2$ : increasing oxidising power
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Solution:
- (b) If acidic nature is high, $K_a$ is high and $pK_a$ is low
\begin{tabular}{lll} & $H_2 O$ & $H_2 S$ \\ $K_a$ & $1.8 \times 10^{-6}$ & $1.3 \times 10^{-7}$ \\ & $H_2 Se$ & $H_2 Te$ \\ $K_a$ & $1.3 \times 10^{-4}$ & $2.3 \times 10^{-3}$ \\ since $pK_a=-\log K_a$ & \end{tabular}
Hence the order of $pK_a$ will be
$H_2 O>H_2 S>H_2 Se>H_2 Te$
