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Working out equilibrium amounts for Kc and Kp

I've been doing Kc and Kp equations for AQA chemistry and I can do them well, but it's just the start I find difficult. That is when they give you the initialnumber of moles of one substance and the number of moles at equilibrium of the same or a different substance.
I really don't understand how to find the equilibrium amounts of the other products, and have basically been just trying to remember the different questions on past papers rather than understanding on whats happening.
My chem teachers have all tried to help me understand but I get really confues when things aren't in the same mole ratio and you have to work out the stoichiometry.
If anyone could help me get my head around this I would be extremely grateful.
Thanks
Reply 1
Avatar for Eau
Eau
15
Well to find the equilibrium amounts, you would need to find the initial amounts and the change in amounts.

It's really hard to explain it all in once. It's best that you post a problem you can't solve/understand.
Reply 2
Avatar for Giggsy11
Giggsy11
OP
2
A 36.8 g sample of N2O4 was heated in a closed flask of volume 16.0dm3. An
equilibrium was established at a constant temperature according to the following
equation.
N2O4(g) - 2NO2(g)
The equilibrium mixture was found to contain 0.180 mol of N2O4
(i) Calculate the number of moles of N2O4 in the 36.8 g sample.
..................................................................................................................................
..................................................................................................................................
(ii) Calculate the number of moles of NO2 in the equilibrium mixture.
..................................................................................................................................
..................................................................................................................................

I understand the first part on how to get the number of moles. Thats just moles=mass/Mr. Seen what to do for the second part but don't understand why.

CO(g) + 2H2(g) CH3OH(g)
A 62.8 mol sample of carbon monoxide was added to 146 mol of hydrogen. When
equilibrium was reached at a given temperature, the mixture contained 26.2 mol of
methanol at a total pressure of 9.50 MPa.


Working out the number of moles for this one I don't paticularly understand either
Thanks
Reply 3
Avatar for Eau
Eau
15
Just multiply (i) by two. The equation says that one mole of N2O4 gives two moles of NO2.

The second question:

Work out, for each species:

(i) initial amounts: 62.8, 146, 0
(ii) change: -x, -2x, +x
(iii) final amounts , 62.8-x, 146-2x, x

x = 26.2, so work out the rest.

The rest you should be able to do.

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