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  4. Equimolar solutions of the following compounds are prepared separately in water. Which will have the lowest pH value ?

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Q. Equimolar solutions of the following compounds are prepared separately in water. Which will have the lowest pH value ?

JEE MainJEE Main 2013Equilibrium

Solution:

Metal halide on hydrolysis with water form corresponding hydroxides.
The basic strength of hydroxide increases as we move down in a group. This is because of the increase in size which results in decrease of ionization energy which weakens the strength of $M - O$ bonds in $MOH$ and thus increases the basic strength.
$\underset{\text{Amphoteric}}{Be(OH)_2} \underset{\text{Weak base}}{Mg(OH)_2}$
$Ca(OH)_2\underset{\text{Strong base}}{Sr(OH)_2}Ba(OH)_2$
Hence, $Ba(OH)_2$ will have lowest pH.