I don't get this q in MCAT 4!!

[Guest ioncannon97]

The question # is 133 for those who have it.

 

The question is: If the molar mass of the oxide is known, what other characteristic of the solution is required to calculate the molarity of its saturated solution?

A. Its density

B. Its mass

C. Its volume

D. Its pH

 

I personally don't see how molarity can be calculated from any of the answer choices but the answer is A. and the explanation from AAMC is:

 

"Molarity is the number of moles of solute per liter of solution. Therefore, two variables are needed to find molarity. One variable is moles or a quantity such as mass that can be converted into moles. The other variable is volume. If we know the number of grams per mL (i.e. the density of the solution), we can multiply by 1000 to get grams per liter and divide by molar mass to get moles per liter of solution. Thus, answer choice A is the best answer."

 

I, however, have a problem with this explanation. Molarity if moles of the OXIDE per liter of SOLUTION. How can we take the DENSITY OF THE SOLUTION in g/L, divide it by molar mass of the oxide, and get MOLARITY? that doesn't make sense because the g in g/L is mass of solution (which includes mass of solvent), not mass of solute!. Consider the following:

 

A dilute solution of, say, HCl with molarity 0.001 M has density of approx 1000 g/L (the density of water since the small amount of solute won't make much difference). Now, by AAMC's logic in the explanation we can find the molarity of this solution (assuming we don't already know its 0.001M) by dividing the density by molar mass of HCl. Which is 1000/36.5 =27.4M which is obviously wrong.

 

Only 20% of the people got this q right making it the most difficult q on the test, so maybe I'm just not catching onto some trick. Can anyone help?

[Guest V]

mol/l = mass/volume divided by mass/mol

 

this gives mol/l = mol/volume (and volume is litres)

 

Thus if you have molar mass ( mass/mol) you only need Mass/volume which is density.

 

The density they speak of IS a) of the solute BUT, of an aqueous solution of the solute.... hence the denisty of the solute and solution are one and the same.

 

YOU WROTE

 

"I, however, have a problem with this explanation. Molarity if moles of the OXIDE per liter of SOLUTION. How can we take the DENSITY OF THE SOLUTION in g/L, divide it by molar mass of the oxide, and get MOLARITY? that doesn't make sense because the g in g/L is mass of solution (which includes mass of solvent), not mass of solute!"

 

Your reasoning would be right if this were a solid phase question.... but it is not. It is an aqueous phase question so it is implied that the solute is dissolved.

 

I really wouldn't get caught up on this... On test day you don't have time to do that. Look, you know B,C and D are DEAD wrong, and A is right if it said density of solute rather than of solution, so circle it and move on. Do you agree?

[Guest rosie7172]

Ok, so I think I have a good way for you to look at the problem... just look at is as simple algebra and cross multiply the units to find out what you are missing:

 

- you have the molar mass- grams solute/moles solute

- you want the molarity- moles solute/L solution

 

so what are you missing?

 

(grams solute) X (A) = (moles solute)

___________ ____ ____________

(moles solute) ( (L solution)

 

- for the units to cancel, (A) must be (L of solution) and ( must be (grams solute).

- The relationship grams solute/L solution is the definition of density.

 

Hope that's an easier way of looking at it.

 

Good luck!

[Guest ioncannon97]

Thx for the responses guys. I think V's thinking is right. Its the only answer that comes close to making sense to me so ill just leave it at that.

[Guest Richardo Xavier]

Always think of inorganic chemistry stuff in terms of Units.

 

For example, don't just think Molarity... just think..

 

"Oh, Molarity... that's Moles of solute per Liter of solvent."

 

That helped me out a ton getting through college chem.

 

-Richie

[Guest dfenst]

I haven't read the other answers, but here's my quickie.

 

You know g/mol. You need mol/L. The reciprocal of g/mol is mol/g (which you want because the numerator of molarity is mol). You need to get rid of the g denominator and replace it with L. How? Multiply your mol/g by g/L. This gives you mol/L. g/L by definition is density (mass/volume). Hope that helped.

[Guest rosie7172]

That's basically what I said... did we go to the same school or something?

[Guest medicator007]

I doubt you are referring to the same HS, as you both got it right?

[Guest ioncannon97]

It seems V was the only one who understood my problem. I didn't have trouble with the units as most of you believe. I just didn't see how you could take the DENSITY OF THE SOLUTION which was grams of solute and solvent per liter and assume that density was only grams of solute per liter and get the molarity. I do understand dimensional analysis so that wasn't the problem. Anyway, its ok, V's advice seemed good enough for me. I should just not think too deeply into q's like this. That only results in trouble.