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# How do you work out the number of moles in an equilibrium mixture? watch

1. Hi!

I was wondering if anyone could help me with a little problem of mine.

First off, here's the question... in question;

"2.0mol of A is mixed with 2.0mol of B and the mixture was allowed to reach equilibrium at 500C. The equilibrium mixture was found to contain 1.00mol of A. Calculate a value for Kp.

2A + B = 3C"

It's not the calculating Kp bit I struggle with, give me a 1:1 ratio equation and I could work through it no problems, but I've always struggled to get my head around the probably simpler method of working out the numbers of moles at equilibrum in an equation where the ratios of products/reactants are not the same.

If it was A + B = C; I know 1mol of A reacted, so 1mol of B must have reacted to produce 1mol of C. Easy. But it's 2A in the equation, so is that 0.5mol of B that reacted? How much C is produced in this 2:3 ratio? I have a feeling 1.5 but I'm a bit lost.

Hope that made sense and that someone can clear this up for me!

Many thanks.
2. (Original post by DRe90)
Hi!

I was wondering if anyone could help me with a little problem of mine.

First off, here's the question... in question;

"2.0mol of A is mixed with 2.0mol of B and the mixture was allowed to reach equilibrium at 500C. The equilibrium mixture was found to contain 1.00mol of A. Calculate a value for Kp.

2A + B = 3C"

It's not the calculating Kp bit I struggle with, give me a 1:1 ratio equation and I could work through it no problems, but I've always struggled to get my head around the probably simpler method of working out the numbers of moles at equilibrum in an equation where the ratios of products/reactants are not the same.

If it was A + B = C; I know 1mol of A reacted, so 1mol of B must have reacted to produce 1mol of C. Easy. But it's 2A in the equation, so is that 0.5mol of B that reacted? How much C is produced in this 2:3 ratio? I have a feeling 1.5 but I'm a bit lost.

Hope that made sense and that someone can clear this up for me!

Many thanks.
so just look at the equilibrium amounts. you have 1 mol of a. As there was initially 2 mol of a 1 mol must have reacted. now look at the molar relationship in the equation. if 1 mol of a reacted then 0.5 mol of b reacted because of the 2 to 1 relationship as you say leaving you with an equilibrium amount of 1.5 mol for b. now in order to find how much C has formed look on the left for the reactant with can divide into that number 3 easily (1 into 3 goes easier than 2 into 3). in this case the reactant is b. So what you do is look at how much b reacted (0.5) mol and because its a 1 to 3 relationship you know 1.5 mols of c formed as you say.

Hope this helps

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