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"omitting" the units

Posted: Mon Jan 13, 2020 4:16 pm
by Ghadir Seder 1G
Professor Lavelle mentioned that when calculating K, you omit the units. What does this mean and why do we do it?

Re: "omitting" the units

Posted: Mon Jan 13, 2020 4:19 pm
by Brian_Ho_2B
We omit the units because in actuality we are supposed to be calculating equilibrium constants and reaction quotients using the activities of each reactant or product of a reaction. Activities are unitless, only scalar values, so it makes sense that calculating a value K with unitless values produces another unitless value. However, for the purpose of this class, activities and concentration/partial pressure values are close enough to the activities' values that we use those to calculate K. We are concerned about the values when we calculate K or Q, not necessarily the units involved.

Re: "omitting" the units

Posted: Mon Jan 13, 2020 4:22 pm
by Chem_Mod
We are technically using chemical activities to calculate K. Activities are unitless, so that makes K unitless. To be be more exact, you should be writing the equilibrium constant by using activities. K equals the activities of products to respective stoichiometric coefficients over activities of reactants to respective stoichiometric coefficients. Keep in mind that the activity of water is 1 when it is the solvent. We use concentrations to calculate K because it is a good enough approximation to activities.

Re: "omitting" the units

Posted: Tue Jan 14, 2020 10:06 pm
by cassidysong 1K
^^as they stated K represents activity which is unitless

Re: "omitting" the units

Posted: Wed Jan 15, 2020 3:32 pm
by Morgan Carrington 2H
You aren't really "omitting" the units. It is just that since all of the units on the values being used to calculate K[sub]c[sub] are the same, they cancel each other out. It is sort of like when doing conversion factors you put the same units in the denominator or numerator in order to cancel them out, but this is completely unintentional.

Re: "omitting" the units

Posted: Thu Jan 16, 2020 1:18 pm
by Kassidy Ford 1I
But in this course often a question will ask for a specific K, like Kc or Kp. In this case, make sure your units match with what the question is asking for. But K does not necessarily have to have this specificity

Re: "omitting" the units

Posted: Sat Jan 18, 2020 6:35 pm
by Matthew Tsai 2H
As mentioned in this thread, because the units in the numerator and the denominator are the same, you can think of K as "unitless" because it just indicates the direction and degree of reaction.

Re: "omitting" the units

Posted: Sun Jan 19, 2020 4:18 pm
by Jocelyn Thorp 1A
You're not really "omitting" units, K is unitless (since it is a ratio, the units "cancel")