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Question:

In which of the following arrangements, the given sequence is not strictly according to the property indicated against it?

H2O<H2S<H2Se<H2Te; increasing pKa values

CO2<SiO2<SnO2<PbO2; increasing oxidising power

NH3<PH3<AsH3<SbH3; increasing acidic character

HF<HCl<HBr<HI; increasing acidic strength

Solution:

The following sequence is not strictly according to the property indicated against it: H2O < H2S < H2Se < H2Te; increasing pKa values. The correct sequence is: H2O > H2S > H2Se > H2Te; decreasing pKa values. In hydrides, as we move top to bottom, acidic nature increases so with the increase in the acidity from top to bottom, the Ka value increases and pKa value decreases.