tough Molality question

[Predentole]

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The molarity of a solution of KOH is 8M and has a density of 1.3 g/mL. The solvent of this solution is unknown and the molality of this solution is

A. 8.00 m
B. 9.39 m
C. 6.15 m
D. depends on the solvent used

Can someone try to answer this?

 

[atlanta213]

hmm, i think it is C.

If this is the answer, i will explain this to you. Let me know

 

[dleodyddlek]

I'll go for D lol
but i'm not 100% sure.
Since Molarity is mols/L you get 8M but molality is mols/Kg. I think you need to know the mass of the solvent as well. I haven't really gone thru any deep thinking process so I can't gurantee it's D lol
let me know if it's the answer. If it is, i'll come back to it and explain more details

 

[Predentole]

Sorry guys, the answer is B 9.39 m

 

[Predentole]

This is what they do but I don't understand.

1L x 1000mL/1L x 1.30g/1mL - 8mol KOH x 56g/1mol = 852g

The molality of the solution = 8mole/0.852kg = 9.39m

I looked at this but still don't get it

 

[oral09]

I think M=n/V. If you devide both sides by d. then you get M/d = n/ V.d . V.d = m(solvent). So m = M/d. plus the number in but dont forget to convert it to Kg

 

[wasabi82]

it seems to make sense after looking at your answer.
Molality is (mol of solute/ kg solvent).
1L x 1000mL/1L x 1.30g/1mL will give you total grams of solution in 1L and 8mol KOH x 56g/1mol = 852g will give you grams of solute in 1L, and the difference b/w these two will be grams of solvent.

therefore 8mol(in 1L)/0.852kg (in 1L) will be the molality.

I hope question like this will not be in real DAT.

 

[PooyaH]

it seems to make sense after looking at your answer.
Molality is (mol of solute/ kg solvent).
1L x 1000mL/1L x 1.30g/1mL will give you total grams of solution in 1L and 8mol KOH x 56g/1mol = 852g will give you grams of solute in 1L, and the difference b/w these two will be grams of solvent.

therefore 8mol(in 1L)/0.852kg (in 1L) will be the molality.

I hope question like this will not be in real DAT.

I just wanna make sure I'm getting this right lol

So we have 8mol of KOH so basically we get:
8ml KOH x (56g KOH/1ml KOH) = 448g KOH...and
we have 1.3g/ml x (1000ml/1L) x 1L = 1300g of solution!...and
1300g of solution - 448g of KOH = 852g or .852Kg of Solvent...and
Since Molality = moles of solute / Kg of Solvent, then
8mol of KOH / .852Kg of Solvent = 9.3

 

[joonkimdds]

I just wanna make sure I'm getting this right lol

yes that's how u do it.