Chemistry Equilibrium MDCAT 2010
PMDC Verified Question 102 of 102
Units of \( K_c \) for the following reaction is:

$$ \text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI} $$
A
\( \text{mol}^2 \text{dm}^{-6} \)
B
No unit
C
\( \text{mol dm}^{-3} \)
D
\( \text{mol}^{-2} \text{dm}^{6} \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option B: No unit
Concept:

The unit of the equilibrium constant \( K_c \) is entirely dependent on the change in the number of moles of gas (\( \Delta n \)) between products and reactants.

Formula:

$$ \text{Units of } K_c = (\text{mol dm}^{-3})^{\Delta n} $$

Solution:

  • First, calculate \( \Delta n = n_p - n_r \).


  • For \( \text{H}_2 + \text{I}_2 \rightleftharpoons 2\text{HI} \), moles of product \( n_p = 2 \).


  • Moles of reactant \( n_r = 1 + 1 = 2 \).


  • \( \Delta n = 2 - 2 = 0 \).


  • Thus, \( (\text{mol dm}^{-3})^0 = 1 \), leaving the constant with no unit.


Why other options are incorrect:

Other options represent units for reactions where there is a net change in gaseous moles (\( \Delta n \neq 0 \)).

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