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In which of the following arrangements the given sequence is not strictly according to the property indicated against it?
A
$CO_2 < SiO_2 < SnO_2 < PbO_2$ : increasing oxidising power
B
HF < HCl < HBr < HI : increasing acidic strength
C
$H_2O < H_2S < H_2Se < H_2Te$ : increasing $pK_a$ values
D
$NH_3 < PH_3 < AsH_3 < SbH_3$ : increasing acidic character
Detailed Solution
Down group 16 the H–E bond becomes weaker, so acid strength increases: $H_2O < H_2S < H_2Se < H_2Te$.
A stronger acid has a larger $K_a$ and therefore a smaller $pK_a$.
So $pK_a$ decreases in the order $H_2O > H_2S > H_2Se > H_2Te$; the sequence given as increasing $pK_a$ is not correct.
The other three sequences are correct.
A stronger acid has a larger $K_a$ and therefore a smaller $pK_a$.
So $pK_a$ decreases in the order $H_2O > H_2S > H_2Se > H_2Te$; the sequence given as increasing $pK_a$ is not correct.
The other three sequences are correct.