[bonggaka]

Hello everyone. Not really sure how to answer these questions. Please help me out. Please. 

Question No. 1

A mixture containing 2.0 g of A and 8.0 g of B is to be separated and the components purified by one crystallization from 100 mL of solvent. Their solubilities are given in the table below.

                           A                                     B
Solvent         cold       hot                  cold             hot

Ethanol          0.5       10.0                 5.0               20.0       
Acetone         5.0       20.0                 0.1               8.0
Water            0.6       15.0                 0.5               10.0


a. What solvent would you choose? Support your answer
b. Theoretically, what is the purity of A and B after one crystallization?


Question No. 2

A mixture of three compounds, A, B, and C is to be separated and purified by crystallization. Their solubilities in g/100mL ethanol are given below:

Solvent           A                B               C

Cold             1.05             5.60           4.20
Hot              20.5             2.3            3.50


a. If a mixture containing 4.0 g of each A, B and C is recrystallized from 100 mL ethanol, which compound would be obtained pure?

b. If the filtrate from (a) is evaporated to 50 mL, which of the compound(s) will separate upon cooling? What is its purity?




P.S. Thanks for the help. I really really need an angel to help me with this problem set. Thanks, really. 

[marcal]

Before I comment, what´s your opinion on the topic? What do you think the answers are?

[bonggaka]

marcal. I am not sure with my answer but for Question 1 I guess it's Water. The best solvent is one that is highly soluble in hot solvent but is almost insoluble in cold solvent. I do not know how to obtain the purity though. As with question 2a, I guess compound A would be obtained pure. I have no idea how to solve letter b though. Sorry. I realy hope you can help me with this. Thanks

[marcal]

A trick in crystallizations is to have clear the relative solubility of one component to the other. Take the different conditions and instead of having absolute solubility, have the ratio instead.

[bonggaka]

thanks. but i didn't get it. i still don't know how to answer the question. really sorry. i've been working with this since morning and I still don't know until now how to do this.

[marcal]

thanks. but i didn't get it. i still don't know how to answer the question. really sorry. i've been working with this since morning and I still don't know until now how to do this.

Go outside, have a walk, drink a coffee and try it again.

Look, take for instance exercice 1. For each condition (hot or cold), calculate the ratio of the solubility (r=aol. B/sol. A). This will give you a better overview of how more soluble is a component versus the other. For example take Ethanol Cold. In this conditions, r=s B/s A = 5.0/0.5=20  It means that (in saturation conditions) for each 20 grams of component B present in the solution there will only be 1 of component A.

Now apply this reasoning to the rest of conditions and the quantities you´ve got and see what you come up with.

[Arkcon]

Like everyone else has said, your table's information is all over the place, and is confusing you.  Weird, I usually tell people with a word problem to "build a table" You have to convert it to a simpler table.  Create ratios, that is, divide one data point by another, to make a simpler table.  Then it will be clear.  Once you decide what to divide by what.  Look closely, what do you need to know?

[bonggaka]

thanks everyone. Got the first one but still unclear to me how to answer Question No. 2 letter b only. And Question No. 1 letter b too. The decision on which solvent was made clear and thanks to both of you. I hope I'm right with my answer. But as with the unanswered questions please do help me out in answering the unanswered questions. Thanks. With much respect.

[fledarmus]

You know how much of each component you are starting with. When you put them into your hot solvent, how much of each of them will dissolve? When you cool the solvent back down, how much of each will stay dissolved? The crystals that you form will theoretically be composed of the difference between the amount of each dissolved in the hot solvent and the amount dissolved in the cold solvent.

[marcal]

When you're done figuring that out, please check one thermodynamic book. The chapter which talks about Raoult's law. When you read it, think about this exercises and everything will make sense.

[orgopete]

I am beginning to wonder if I know how to do this. Ratio? Why? Given the set up of question 1), I looked at the solvents and tried to determine if I could dissolve A or B in the cold solvent. I thought I could dissolve A.

For 1b), I am going to rewrite this so you know what I am going to calculate. "What is the ratio of A and B in the filtrate? With the solvent I chose, all of A and the soluble part of B.

If I did 1) correctly, then I would have thought I could do 2) in the same way, but the table looks wrong. More dissolves in cold than hot? Can't be.