textbook 5I.11

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Lucy Wang 2J
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textbook 5I.11

Postby Lucy Wang 2J » Mon Jan 18, 2021 9:15 pm

A reaction vessel of volume 0.500 L at 700. K contains
1.20 mmol SO2(g), 0.50 mmol O2(g), and 0.10 mmol SO3(g). At
700. K, Kc 5 1.7 3 106
for the equilibrium 2 SO2(g) 1 O2(g) ∆
2 SO3(g). (a) Calculate the reaction quotient Qc.

I calculated Qc=(.1)^2/[(.5)(1.2)^2]=.0139, however the answer key says that Qc=6.9. Can someone please explain what went wrong?

John Pham 3L
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Re: textbook 5I.11

Postby John Pham 3L » Mon Jan 18, 2021 10:38 pm

You still need to convert from mmol to mol (1 mmol = 10^-3 mol)
Make sure to also calculate the molarity by dividing by 0.500 L before plugging into the Kc equation.

Stephanie Zhang 2K
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Re: textbook 5I.11

Postby Stephanie Zhang 2K » Mon Jan 18, 2021 11:47 pm

Yes, I agree with the response above! As a general rule, always make sure to double check the units of the quantities given. Since you need the concentrations of the species in order to find Qc, you have to make sure each value you plug into the general Qc equation has the units of mol/L, which is also just mol x L^-1 or M.
That means for each value given, you have to convert from mmol to moles, and generally when there's a 'm' in front of the unit that usually means 'milli', and for a given unit x, 1mx= 10^-3x. For example, 1mg= 10^-3 g.
After that, you just divide by the volume 0.500 L to get the units of M, and then from there you can plug the values into the Qc equation. With other problems, also make sure to check the given units of the volume, because sometimes they're given as mL.

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