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# Equilibrium concentration question HELP? watch

1. So here's the question and the working out given in the book

And the working out
Attachment 509279509281509300
My question is where did the change/mol values come from I'm very confused (I.e -1.20 and -0.60)

Thanks
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2. The Change/Mol is the change of the total moles for the reaction from it's initial point to the point it has reached equilibrium.
Initially, NO has 2 mol concentration and O2 has an initial concentration of 1 mol. As we are not told the initial concentration of NO2 we assume this is 0.

We are told that at equilibrium the concentration of NO2 equals 1.2 mol. As a result from it's initial concentration of 0, NO2 has become +1.2 mol. Therefore, this is +1.2 change/mol.

To calculate the moles of NO and O2 at equilibrium we must use the value we are given for NO2. From the equation we know that 2 moles of NO2 are equal to 2 moles of NO. Therefore, as they are a 1:1 mole ratio, we can take the mole value of NO2 at equilibrium away from the initial value of NO to calculate its concentration at equilibrium. 1.6-1.2= 0.4 moles of NO at equilibrium. Therefore, the concentration of NO has decreased by 1.2. Hence the value of -1.2 change/mol.

From the equation it is evident that 2 moles of NO2 are equal to one mole of O2. As a result to calculate the moles of O2 at equilibrium we must divide the moles of NO2 at equilibrium by 2, and take that away from the initial mole value of O2. Therefore, 1.2/2=0.6. 1.4-0.6=0.8 moles of O2 at equilibrium. As we have subtracted 0.6 the change/mol = -0.6.

I hope I have been able to help! Any more questions, Just ask!
3. They change because you have to use the molar ratios in the equation to work out how many moles you have of your reactants left @ equilibrium. If we made 1.2 moles of NO2 then as per the molar ratio we had to use 1.2 moles of NO and 0.6 moles of O2, taking these values away from our initial values we are left with 0.4 (1.6-1.2) moles of NO and 0.8 moles (1.4-0.6) of O2.

Then you can / by your vol to get conc and punch the values into the equation.

This video will really help: https://www.youtube.com/watch?v=tT-2xk9ZG_A

Hope that helps.

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