# Is molarity the same as probability?

Frigus
In the keq derivation intuition video mr sal Khan relates molarity to probability and it doesn't makes sense to me as molarity can also be more than 1 and probability cannot.
Can you please tell me how he relates probability to molarity.

Thanks

Mentor
Ugh, my trust in Khan's Academy just took a deep dive Broadly speaking he never claims the probability to equal molarity, he says they are related by which he means they are directly proportional - there is some scaling factor that converts the concentration into probability. Imagine you have a mixture of 1 M N2 and 2 M H2 - if you draw a random molecule from the mixture probability that it is nitrogen or hydrogen definitely depends on their concentrations, but is never equal to them.

However, his "explanation" is convoluted to the point of being completely useless, plus the idea of "probability that things are going to react just because they happen to be in the same place" doesn't make sense to me.

• Frigus and jim mcnamara
Gold Member
the idea of "probability that things are going to react just because they happen to be in the same place" doesn't make sense to me.
This refers to the pre-exponential factor in the Arrhenius equation. Just because two molecules collide, doesn't mean they'll react. They have to have sufficient energy, be in the correct orientation, be in the correct quantum vibrational and electronic states, etc. So given a collision between two molecules, there is a certain probability that they'll react with each other based on those considerations.

• jim mcnamara
Frigus
Broadly speaking he never claims the probability to equal molarity, he says they are related by which he means they are directly proportional - there is some scaling factor that converts the concentration into probability. Imagine you have a mixture of 1 M N2 and 2 M H2 - if you draw a random molecule from the mixture probability that it is nitrogen or hydrogen definitely depends on their concentrations, but is never equal to them.
Thanks sir,
Now my misconception about it is cleared.

Mentor
This refers to the pre-exponential factor in the Arrhenius equation. Just because two molecules collide, doesn't mean they'll react. They have to have sufficient energy, be in the correct orientation, be in the correct quantum vibrational and electronic states, etc. So given a collision between two molecules, there is a certain probability that they'll react with each other based on those considerations.

Yes, but he calculates "probablity" using just the presence of molecules in dV and not saying anything about the fact some of the collisions are inactive. For me that's - from pedagogical point of view - just replacing one nonintuitive information with another, I don't see how it can help in understanding the idea of equilibrium.

In other words: IMHO he doesn't explain anything, just does some convoluted hand waving for 15 minutes.

Gold Member
I didn’t get that; he mentioned different configurations several times throughout the video. I guess it doesn’t bug me because I do enough Monte Carlo type simulations where reaction rates have to be translated into probabilities. To each his own.

Frigus
Can anyone please tell me how he derived Keq by equating probabilities,i have been taught that we equate rates but not probability.

Mentor
The probability of them reacting is directly proportional to the rate.

• Frigus
Frigus
The probability of them reacting is directly proportional to the rate.
Thanks,
So we can get rate by multiplying probability with some constant.

Mentor
Yes.

• Frigus