Vapor Pressure

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[LP]

Really need help with this:
Alcohol (for example CH3-OH) dissolves in pure water to give a solution that boils at a lower temperature than pure water.Assume pure water boils at 100 degrees Celsius.
Ordinary table salt  (NaCI) dissolves in pure water to give a solution that boils at a higher temperature than pure water. Explain these facts from the standpoint of vapor pressure.
thank you

[spock]

Adding a solute has the effect of raising the boiling point by lowering vapor pressure.

In the alcohol/water mixture the water lowers the vapor pressure (and therefore the bp) of the alcohol (which boils before water), while in the water/salt mixture the salt lowers the the vapor pressure of the water.

[LP]

Thank you so much.... I have another question -do you have time to help me
A reaction occurs when water is added 4.267 grams of UF6. The only products are 3.730 grams of a solid containing only uranium and fluorine together with 0.970 gram of a gas. The gas is 95% fluorine by mass and the remainder is hydrogen.
1.From this data determine the empirical formula of the gas that contains only fluorine and hydrogen.
2.What fraction of the fluorine of the original compound UF6 is in the solid and what fraction in the gas after the reaction?
3.What is the empirical formula of the solid product
4 Write a balanced equation for the reaction between UF6 and H20. Assume that the empirical formula of the is the true formula

[spock]

Well, from the percent values (95% F, 5% H) determine the empirical formula for the compound)

Now it gets a little stickier.  There is an error in the question it is impossible to solve the rest of the problem as written.  According to the question you have U, F, H, and O atoms as reactants and U, F, H atoms as products.  Can't happen.

Are you sure that the solid product doesn't contain U, F, and O?

[LP]

Shoot! you are right I accidentally left out oxygen....wow thank you

Thank you so much.... I have another question -do you have time to help me
A reaction occurs when water is added 4.267 grams of UF6. The only products are 3.730 grams of a solid containing only uranium and OXYGEN and fluorine together with 0.970 gram of a gas. The gas is 95% fluorine by mass and the remainder is hydrogen.
1.From this data determine the empirical formula of the gas that contains only fluorine and hydrogen.
2.What fraction of the fluorine of the original compound UF6 is in the solid and what fraction in the gas after the reaction?
3.What is the empirical formula of the solid product
4 Write a balanced equation for the reaction between UF6 and H20. Assume that the empirical formula of the is the true formula

[spock]

1) Calculate moles of UF6.
2) Calculate moles of HF
3) Calculate initial moles of F in UF6 ( 6 x ans. 1)
4) Calculate moles of F in HF (= ans. 2)
5) Calculate moles of F in solid product (ans. 3 - ans. 4)
6) Calculate the mass of U in the solid from ans. 1 (Formula indicates that there is 1 mole of U in each formula unit of UF6) and all of it went into the solid product.
7)  Calculate the mass of F in the solid using ans. 5
  Using the total mass of the solid and the masses of U and F, calculate the mass of O in the solid.
9) Find the moles of O in the sample.
10) Determine the smallest whole number ratio of U, O, and F in the solid.

[LP]

How do I calculate the moles for HF

[LP]

I got the answer to moles to HF I don't know what question you answered sorry I am confused.... there were 4 parts to the question and you have ten steps however I do not know what step goes to which 4 parts of the question I asked...sorry

[spock]

I told you how to do #1 in a previous post.  The answer is HF.

The ten steps I gave you yield the empirical formula (Your Question 3), but the information you find as you go through the ten steps will enable you to find the answers to your questions #2 and #4.

Work through my ten steps and when you have the answers to them write back if you still can't figure out Questions 2 and 4.

[LP]

Still working on it...very frustrated...can you do it step by step for me....sorry I have now been working on it for an hour....thanks

[spock]

You need to work through the steps:

Step 1: Find the number of moles of UF6    (moles = grams in sample/formula mass)

Come back with an answer and I'll show you the next step.

[LP]

ok for the moles of UF6 i got 0.01446 is that right? then what do i do next?

[spock]

I get .0121 moles   (4.267 grams of UF6/ formula wt of UF6)
What did you use for the formula weight of UF6?
(Please show your work so I can tell where the problem is)

[LP]

oh I see what I did wrong... Now i got 4.267g UF6 divided by 352 equals 0.01212 moles of UF6.  then what do i do after that?

[spock]

So now we find the moles of the gas which is HF (I explained how to determine the formula of the gas in an answer to one of your other posts of this question)

There are .970 grams of the gas (which is HF), calculate the number of moles of HF.