gChem - molality question

[Dentist2be]

Could anyone please explain how to do this? I don't seem to understand how you get the first four multiplications. (The answer is B)
Thanks for your help!


Q. What is the molality of 85% phosphoric acid, H3PO4, if the density of the solution is 1.70 g/ml?

A. 1000 * (1.70/1) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
B. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
C. 1000 * (1/1.70) * (100.0 / 15.0) * (1/1.70) * (85.0 / 100.0) * (1/98) m
D. 1000 * (1/1.70) * (15.0 / 100.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
E. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (100.0 / 85.0) * (1/98) m

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[Gasedo]

Could anyone please explain how to do this? I don't seem to understand how you get the first four multiplications. (The answer is B)
Thanks for your help!


Q. What is the molality of 85% phosphoric acid, H3PO4, if the density of the solution is 1.70 g/ml?

A. 1000 * (1.70/1) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
B. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
C. 1000 * (1/1.70) * (100.0 / 15.0) * (1/1.70) * (85.0 / 100.0) * (1/98) m
D. 1000 * (1/1.70) * (15.0 / 100.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
E. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (100.0 / 85.0) * (1/98) m

I know, I had problem with this too when I took the DAT Achiever test.
But if you really don’t have a clue how to solve it just think only at the units. You need to get m in the end.
So replace the units in each equation:
g * 1/g *g/g * g/1 *g/g * mol/g = mol

You will be left in the end only with m. Everything else will cancel.
If you try it with any other choice you’ll see that you cannot cancel everything besides m.
I know this will take a bit more time, but fortunately for this question you need to do only to trials, the second answer is right.
If the last answer was right, then you should have worked longer.
Hope this helped, and maybe somebody else is willing to explain in more detail.

 

[seoul]

I think this question has also been well dicussed through the following thread:

http://forums.studentdoctor.net/showthread.php?t=225090

 

[travelbug73]

Could anyone please explain how to do this? I don't seem to understand how you get the first four multiplications. (The answer is B)
Thanks for your help!


Q. What is the molality of 85% phosphoric acid, H3PO4, if the density of the solution is 1.70 g/ml?

A. 1000 * (1.70/1) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
B. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
C. 1000 * (1/1.70) * (100.0 / 15.0) * (1/1.70) * (85.0 / 100.0) * (1/98) m
D. 1000 * (1/1.70) * (15.0 / 100.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
E. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (100.0 / 85.0) * (1/98) m

An easy way to look at it would be:

molality as you know = moles of solute/kg of solvent

Since the problem states that the solution is 85% H3PO4, you can assume that out of 100g of solution, 85g is H3PO4 (solute) and 15g is water (solvent). The solvent needs to be converted to kg from g and and you need to convert solute to moles using the M.W. of H3PO4 = 98.

Moles H3PO4 = 85/98
kg solvent = 15/1000

Threfore, the simplest eq. would be:

m = (85/15) * (1000) * (1/98)

In your answer choices you were given the density but that is really not needed, hence, you need to pick the answer choice where the density cancels out but the above eq. still holds true and that would be (B).

 

[Dentist2be]

I think this question has also been well dicussed through the following thread:

http://forums.studentdoctor.net/showthread.php?t=225090

Hey, thanks for the info! (nice nickname though )

An easy way to look at it would be:

molality as you know = moles of solute/kg of solvent

Since the problem states that the solution is 85% H3PO4, you can assume that out of 100g of solution, 85g is H3PO4 (solute) and 15g is water (solvent). The solvent needs to be converted to kg from g and and you need to convert solute to moles using the M.W. of H3PO4 = 98.

Moles H3PO4 = 85/98
kg solvent = 15/1000

Threfore, the simplest eq. would be:

m = (85/15) * (1000) * (1/98)

In your answer choices you were given the density but that is really not needed, hence, you need to pick the answer choice where the density cancels out but the above eq. still holds true and that would be (B).

That was nice and quick! I like that. Thanks for your help. I appreciate it.