"omitting" the units

Moderators: Chem_Mod, Chem_Admin

Ghadir Seder 1G
Posts: 134
Joined: Sat Sep 14, 2019 12:17 am

"omitting" the units

Postby Ghadir Seder 1G » Mon Jan 13, 2020 4:16 pm

Professor Lavelle mentioned that when calculating K, you omit the units. What does this mean and why do we do it?

Posts: 221
Joined: Fri Aug 09, 2019 12:16 am

Re: "omitting" the units

Postby Brian_Ho_2B » Mon Jan 13, 2020 4:19 pm

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.

Posts: 19138
Joined: Thu Aug 04, 2011 1:53 pm
Has upvoted: 820 times

Re: "omitting" the units

Postby Chem_Mod » Mon Jan 13, 2020 4:22 pm

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.

cassidysong 1K
Posts: 100
Joined: Fri Aug 30, 2019 12:15 am

Re: "omitting" the units

Postby cassidysong 1K » Tue Jan 14, 2020 10:06 pm

^^as they stated K represents activity which is unitless

Morgan Carrington 2H
Posts: 54
Joined: Wed Nov 14, 2018 12:22 am

Re: "omitting" the units

Postby Morgan Carrington 2H » Wed Jan 15, 2020 3:32 pm

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.

Kassidy Ford 1I
Posts: 103
Joined: Sat Aug 17, 2019 12:16 am
Been upvoted: 1 time

Re: "omitting" the units

Postby Kassidy Ford 1I » Thu Jan 16, 2020 1:18 pm

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

Matthew Tsai 2H
Posts: 101
Joined: Wed Sep 18, 2019 12:20 am

Re: "omitting" the units

Postby Matthew Tsai 2H » Sat Jan 18, 2020 6:35 pm

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.

Jocelyn Thorp 1A
Posts: 103
Joined: Wed Sep 18, 2019 12:20 am

Re: "omitting" the units

Postby Jocelyn Thorp 1A » Sun Jan 19, 2020 4:18 pm

You're not really "omitting" units, K is unitless (since it is a ratio, the units "cancel")

Return to “Ideal Gases”

Who is online

Users browsing this forum: No registered users and 1 guest