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# Which of the following compounds is the most polar: H$_2$O (l), H$_2$S (g), HF (l), HCl (g) ?

$\begin{array}{1 1} HF(1) \\ H_2S(g) \\ H_2O\;(l)\\ HCl(g) \end{array}$

All the compounds given above contain one element same that is hydrogen and its electronegativity is 2.20.
We should then consider the electronegativity of the other elements which is as follows:
$\begin{array} {llll} F & O & Cl & S\\ 3.98 & 3.44 & 3.16 & 2.56 \\ \end {array}$
The most electronegative element is fluorine. So, the electronegativity difference in HF is the most $\rightarrow$ HF(g) is the most polar.
edited Jul 24, 2014