Question

In which one of the following arrangements the given sequence is not strictly according to the properties indicates against it ?

<!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--> AIPMT 2012

• A. $\begin{array}{ccc}N{H}_3<P{H}_3<As{H}_3<Sb{H}_3& :& \text{Increasing acidic charcter}\end{array}$
• B. $\begin{array}{ccc}{H}_{2}O<H_{2}S<H_{2}Se<H_{2}Te& :& \text{Increasing}p{K}_a\text{values}\end{array}$
• C. $\begin{array}{ccc}HF<HCl<HBr<HI& :& \text{Increasing acidic charcter}\end{array}$
• D. $\begin{array}{ccc}C{O}_2<Si{O}_2<Sn{O}_2<Pb{O}_2& :& \text{Increasing oxidizing power}\end{array}$

Answer 1

The correct option is B $\begin{array}{ccc}{H}_{2}O<H_{2}S<H_{2}Se<H_{2}Te& :& \text{Increasing}p{K}_a\text{values}\end{array}$

The elements of group 16 form compounds with hydrogen in which 2 atoms of hydrogen are bonded to one atom of the element of group 16. It is observed that the acidic character increases as we move down the group. So, the $K_a$ value increases down the group and thus the $p{K}_a$ value decreases down the group in the same order. So, the order of $p{K}_a$ values will be:

$H_{2}O>H_{2}S>H_{2}Se>H_{2}Te$
Option (a) is correct