If 5 mol of ethanol are added to 10 mol of water, by how much does the vapor pressure of the water change?
A. It decreases by 33%.
B. It decreases by 50%.
C. It decreases by 56%.
D. It decreases by 67%.
The correct answer is A. The explanation reads "according to Raoult's law, the vapor pressure of a solution where the solute is miscible within the solution like here is directly proportional to the mole fraction of the solvent in the solution."
I understand that since the mole fraction of water decreases by 33% after the additional of the 5 mol ethanol, the partial pressure of water decreases by 33%.
So I am wondering if the explanation I quoted is referring to the decrease in the partial pressure of water or the total pressure of the solution. Using Raoult's Law, P(total) = P(water)*X(water) + P(ethanol)*X(ethanol), so the ethanol has to contribute some pressure to the total pressure. So the new total pressure after the addition of ethanol should not decrease by more than 33% from the original pressure--only the partial pressure of water decreases by 33%, but the added partial pressure of ethanol will bring the total pressure back up a little bit, is that correct?
A. It decreases by 33%.
B. It decreases by 50%.
C. It decreases by 56%.
D. It decreases by 67%.
The correct answer is A. The explanation reads "according to Raoult's law, the vapor pressure of a solution where the solute is miscible within the solution like here is directly proportional to the mole fraction of the solvent in the solution."
I understand that since the mole fraction of water decreases by 33% after the additional of the 5 mol ethanol, the partial pressure of water decreases by 33%.
So I am wondering if the explanation I quoted is referring to the decrease in the partial pressure of water or the total pressure of the solution. Using Raoult's Law, P(total) = P(water)*X(water) + P(ethanol)*X(ethanol), so the ethanol has to contribute some pressure to the total pressure. So the new total pressure after the addition of ethanol should not decrease by more than 33% from the original pressure--only the partial pressure of water decreases by 33%, but the added partial pressure of ethanol will bring the total pressure back up a little bit, is that correct?
