Question
Write the equilibrium constant expression for the decomposition of baking soda. Deduce the unit of KC from the above expression.

Answer

$2 NaHCO _{3( s )} \rightleftharpoons Na _2 CO _{3( s )}+ CO _{2( g )}+ H _2 O _{( g )}$
$K _{ C }=\frac{\left[ Na _3 CO _{2 (s) }\right)\left( CO _{2(g)}\right)\left(H _2 O _{( g )}\right)}{\left( NaHCO _{3( s)}\right)^2}$
The expression for equilibrium constant does not contain the concentration of pure solids.
$K _{ C }=\left( CO _{2( g )}\right) {\left( H _2 O _{( g )}\right) }$
$\text { Units of } K _{ C } =\left( mol dm ^{-3}\right)\left( mol dm ^{-3}\right)$
$ = mol ^2 dm ^{-6} \text {}$

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