# Raoult's Law vs Dalton's Law

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#### sugarbabee0

##### Full Member
10+ Year Member
Whats the difference???

From what I got so far, one deals with partial pressure and the other with vapor pressure... Aren't they the same equation? How do you know which to use when?

raoult's law lets you determine pressures based on mol fractions of stuff in solution. dalton's is just gases i think. yes they are very similar. both are saying the sum of the partial pressures is the total pressure assuming nothing is reacting. raoult lets you get the partial pressure by knowing the mol fraction in solution and the vapor pressure of your target if it were pure.

Raoult's Law: Allows you to determine the partial pressure a liquid in a volatile mixture.

The other, Daltons - Simply allows you to sum individual partial pressures to find the whole pressure.

ie:Here's an example illustrating the use of both in one problem! Neat huh?
Note: you may have a better way of solving this but I made this up (right now quite literally) to illustrate the use of both equations.

Example:
50-50 mixture (by weight) of A and B (MW A--> 4g/mol; MW B-->2g/mol)

Total pressure is 600 torr, what is the partial pressure of A?

No mole fractions, but dont freak. In cases like these, pick any number (preferably one that is round and small).

I'll pick a total of 4 grams. That is the whole sample of gases weighs 2 grams.
What does that mean? --> 2g A and 2g B right?

2g A (1mol/4g) = 0.5 mol A
2g B (1mol/2g) = 1 mol B

We want Partial Pressure of A, but! wait for it...wait for it...bam! we need to consider daltons law now.

P(of A)+P(of B)=600

And we know that the partial pressure of A, that is P=X(subscript of A)P.
So we can plug X(of A)P in for P(of A) and likewise for P(of B).

Now you would get

1/3P(ofA) + 2/3P(of B) = 600

Since these numbers are small, and you could probably guess what P(of A) is, and you wouldn't need to solve using two equations.

Just so you know,

P(of A) is 200 while P(of B) is 400

I never came across a problem that was much more complicated than that when I was studying for the MCAT concerning partial pressures and total pressures. And quite literally, the numbers in this made up problem are the caliber you would see on test day.

Good Luck.