1
$\begingroup$

I am confused between Raoult's law and Dalton's law of partial pressure as they state completely different things about same thing...

Raoult's law claims $p = P \cdot x$
where $p$ is the partial pressure of a gas, $P$ is the pressure of the same gas at the same temperature and volume and $x$ is the mole fraction.

Dalton's law says $p = p_\text{total} \cdot x$
where $p_\text{total}$ is the total pressure of the mixture.

$\endgroup$
1
$\begingroup$

I think you are forgetting that Rault's law is used for ideal Liquid mixtures while Dalton's law is for non-reacting ideal gases. They look similar due to basic assumption of ideal interactions.

In Dalton's law partial pressures add up (PT=PA+PB), but in Rault's law since liquid has to evaporate from a surface which is approximately shared based on mole fraction their vapour pressure do not simply add up and is averaged based on mole fraction PT=xAPA+xBPB.

$\endgroup$
1
$\begingroup$

In Raoult's Law, $p$ is the partial pressure of the vapor in equilibrium with some liquid which has vapor pressure of $P$. $x$ is the molar fraction of this liquid (it can be mixed with other liquid or solid.

$\endgroup$
0
$\begingroup$

Raoult's law states that the partial vapor pressure of a substance is equal to the total vapor pressure of the solvent multiplied by the mole fraction of the substance. This also takes into account things like ion dissociation in a solution [Van't Hoff factor].

Dalton's law of partial pressures says that in a mixture of non-reacting gases, the total pressure of these gases is equal to the sum of the partial pressures of the individual gases (where the partial pressure of a given gas is calculated by multiplying the mole fraction of that gas by the total pressure).

Therefore, Raoult’s Law deals with vapor pressures while Dalton’s Law deals with non-reacting gases. However, the sum of pressures given by Raoult’s Law will results in the total amount of pressure of the [vapor] solution (which follows Dalton’s Law). So Dalton’s Law still holds in that the sum of vapor pressures is the total vapor pressure.

$\endgroup$
0
$\begingroup$

You are using the symbol x to describe two different things. In Raoult's law, $p = Px$, where x is the mole fraction of the substance in the liquid phase. In Dalton's law, $p =P_{tot}y$, where y is the mole fraction of the same substance in the vapor phase. So, combining these, you have $P_{tot}y=Px$, which is really Raoult's law.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.