ACHIEVE week 1, q #2

[EvaHeinrichs3C]

This is what my question looks like on achieve:
At a certain temperature, 0.720 mol SO3 is placed in a 1.50 L container.

2SO3(g)↽−−⇀2SO2(g)+O2(g)

At equilibrium, 0.110 mol O2 is present. Calculate Kc.

I filled out the ICE table and got final amounts of O2= 0.110, SO2= 0.220, and SO3= 0.5, I then divided these each by 1.5(liters) to convert them to molar concentrations. Resulting in O2= 0.73, SO2= 0.146, and SO3= 0.33. Then plugged these values into the Kc equation.

Where am I going wrong here?!

[905783577]

First off, always calculate the molarities before starting your ICE table. Could you send how you set up the equation for K? Did you make sure to account for the coefficients in you equation?

[Maggie Black 1C]

I'm not sure where you're going wrong, but I can offer a few steps that might help.

First, don't forget the exponents when you're calculating concentration if there is more than one mole of a substance (in this case, it would be for SO₃ and SO₂. When you set up your ICE table, you should calculate the molar concentrations first. Also, don't forget to pay attention to the coefficients as well, in addition to whether or not you're adding or subtracting x.

Hope one of these things clear it up for you.

[Natalie Flores 1F]

Hi! My values are the same except my final O2 was 0.140 moles! I'll walk you through my work.
1. My initial values were 0.48M for SO3 (.720moles/1.5L), and 0 for both SO2 and O2. (I found it easier to just convert moles to M first)
2. Since we know the final concentration of O2 is 0.140 moles (=0.093 M) (or 0.110 moles=0.073 M in your case) we know that the change is +0.093 M (for you, 0.073M) between initial O2 and final O2.
3. Using the stoichiometric coefficients, we know that the change in 2SO2 is 0.093x2=0.186 (yours would be 0.073x2=0.146) - we multiply by 2 to account for the stoichiometric coefficient. Similarly, the change between initial and final 2SO3 would be -0.093x2=-0.186 (yours would be -0.073x2=-0.146) - we multiply by 2 to account for the stoichiometric coefficient AND it's negative change because the reactant is decreasing to make products.
4. Once you have the initial values and change values for all the reactants and products, you add the initial values and the change to get your final values at equilibrium in M.
5. Plug your values into the Kc equation ([products]/[reactants] - don't forget the stoichiometric coefficients) to get your Kc value.

Hope this helps!!

[Alice Guey 1B]

I am not sure if this was a typo or a miscalculation, but your final amount of O2 seems to be missing a 0 after the decimal point. Instead of 0.73M it should be 0.073M. Try plugging these values into the Kc equation and see if it makes a difference!