So I was looking around yahoo answers to learn how to do some vapor pressure problems until I saw this question and the guy did some complex calculations to get the answer. I was wondering if for this problem, there's a way to use Raoult's law to solve because I recall that it had something to do with vapor pressure. Can someone please redo this problem using Raoult's law if it is possible? Thank you so much in advance. I'm just so confused and I really want to know if a shorter way is possible.
Here is the link to the problem
http://answers.yahoo.com/question/index?qid=20120131221658AAa3KBs
Here is the link to the problem
http://answers.yahoo.com/question/index?qid=20120131221658AAa3KBs
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Hi Jennifer!
You can definitely use Raoult's law to solve that problem. Just as a quick summary, Raoult's law basically states that whenever a solute is added to a solvent, the solvent's vapor pressure will decrease. This can be represented by the equation: P = A times X where P represents the partial pressure of a component, A represents the vapor pressure of the pure component and X represents the mole fraction of the component.
So let's try to plug all our variables into the pressure equation:
691.4 mmHg = (760 mmHg)(x)
Notice how we need to find x, the mole ratio for the solvent or water. We are given the mass of water (174.1g) so just convert it to moles and then place it in the numerator. Our goal now is to find the total moles of the solution because then we can subtract the moles of water from it to find the moles of NH4Cl added. (174.1g)(1 mole/18.02g) = 9.666 mols of water. So now let's set a variable, y, to represent the total moles of the solution and solve for it.
691.4 mmHg = (760 mmHg)(9.666 mols H2O/y mols)
0.9097 mmHg = (9.666 mols H2O/y mols)
y = 10.63 mols.
Now if the total moles is 10.63, then 10.63 - 9.666 will give us the moles of NH4Cl. The amount of moles is then 0.96. However, we are dealing with a colligative property and so we must be careful about the actual amount of solutes we are adding. NH4Cl is assumed to dissociate completely in the problem and so if it creates NH4+ and Cl- and the total amount of moles of those ions is 0.96, then we are actually only adding 0.48 moles of NH4Cl (just divide by two because 1 mole of NH4Cl makes 2 moles of ions).
Lastly, we can use molar mass to convert the moles of NH4Cl to grams. (0.48 moles of NH4Cl)( 53.49g/1 mole) = 25.64g of NH4Cl or just 26g for significant figures. This answer is definitely close to the actual answer and so I would assume this to be correct.
Now in conclusion, it is possible to use Raoult's law to solve this equation however, it seems that the method used in the link you posted is still much more precise, despite the length and complexity. I would ultimately recommend learning that method of solving this problem but nonetheless, Raoult's law still gives us a close answer!
I hope this helped and feel free to ask more questions! :)
You can definitely use Raoult's law to solve that problem. Just as a quick summary, Raoult's law basically states that whenever a solute is added to a solvent, the solvent's vapor pressure will decrease. This can be represented by the equation: P = A times X where P represents the partial pressure of a component, A represents the vapor pressure of the pure component and X represents the mole fraction of the component.
So let's try to plug all our variables into the pressure equation:
691.4 mmHg = (760 mmHg)(x)
Notice how we need to find x, the mole ratio for the solvent or water. We are given the mass of water (174.1g) so just convert it to moles and then place it in the numerator. Our goal now is to find the total moles of the solution because then we can subtract the moles of water from it to find the moles of NH4Cl added. (174.1g)(1 mole/18.02g) = 9.666 mols of water. So now let's set a variable, y, to represent the total moles of the solution and solve for it.
691.4 mmHg = (760 mmHg)(9.666 mols H2O/y mols)
0.9097 mmHg = (9.666 mols H2O/y mols)
y = 10.63 mols.
Now if the total moles is 10.63, then 10.63 - 9.666 will give us the moles of NH4Cl. The amount of moles is then 0.96. However, we are dealing with a colligative property and so we must be careful about the actual amount of solutes we are adding. NH4Cl is assumed to dissociate completely in the problem and so if it creates NH4+ and Cl- and the total amount of moles of those ions is 0.96, then we are actually only adding 0.48 moles of NH4Cl (just divide by two because 1 mole of NH4Cl makes 2 moles of ions).
Lastly, we can use molar mass to convert the moles of NH4Cl to grams. (0.48 moles of NH4Cl)( 53.49g/1 mole) = 25.64g of NH4Cl or just 26g for significant figures. This answer is definitely close to the actual answer and so I would assume this to be correct.
Now in conclusion, it is possible to use Raoult's law to solve this equation however, it seems that the method used in the link you posted is still much more precise, despite the length and complexity. I would ultimately recommend learning that method of solving this problem but nonetheless, Raoult's law still gives us a close answer!
I hope this helped and feel free to ask more questions! :)