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In the homework problem 5I.19 it asks for the equilibrium constant for the reaction of H2+I2<--->2HI if initially there is 0.4 mol of H2 and 1.6 mol of I2 in a 3.0 L container. They say that 60% of the hydrogen has reacted. When you are doing the ice table for the problem I thought that since the hydrogen is 60% reacted, I should take 60% of the mols of H2 before calculating the concentration but instead you're supposed to take 60% of the initial concentration that you calculate. I was wondering why you would take 60% of the concentration rather than the mol.
When you are writing out your ICE table, you want to find the concentration of reactants and products at equilibrium. For your initial concentration, you would calculate 0.4 mol/3 L H2 to find the initial concentration of H2 which is 0.133 mol/L. For the change in concentration, you would multiply 0.133 mol/L by 0.6 since only 60% of H2 reacts, to get 0.08 mol/L. You are going to use the concentration instead of the mols because the number of moles of solute stay the same in a reaction.
Equilibrium constants are calculated using concentrations (or partial pressures, depending), so you could find the number of moles of reactants and product at equilibrium for this reaction, but you would have to convert them to concentrations in order to find the equilibrium constant.
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