gen chem kaplan question

[jvss123]

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100 g of NaCl is dissolved in 3.5L of water at room temperature. What is the molarity of the resulting solution?

A) 58/35
B) 35/58
C) (3.5 x 58)/100
D) 100/ (3.5 x 58)
E) None of the above.

I know how to do the calculation for this question so I was stuck between two answers (D and E). I ended up choosing E b/c i was under the impression that molarity = moles of solute/liters of solution (NOT SOLVENT, which in this case is water). Am i on the right track? Please explain...

 

[UndergradGuy7]

I would say the answer is D. You can assume in this problem that NaCl does not affect the volume of the solution that much and therefore we don't have to take it into account.

But you are right in the definition molarity = moles of solute / Liters of solution. Where solution is volume of solvent + solute.

Molality is moles solute / kg of solvent.

 

[jvss123]

so then how do you know when to make such an assumption... b/c theres is a another similar question where the answer is "cannot be determined"

If 11 g of CaCl2 is added to 100ml of water, what is the molarity of the solution?
a) 5M
b) 9M
c) 0.9M
d) Cannot be determined <-- CORRECT ANSWER

 

[UndergradGuy7]

Must be an error in the solution, unless someone else can explain?

 

[dentalpath]

I think the answer to the question is E because when i solved this I came up with:
Molarity = no of moles/ Liters of solution
no of moles = weight of solute in grams/atomic weight
which will give Molarity = 100g / 58 / 3.5L
which can also be written as:
(100 x 3.5)/58

 

[jvss123]

dentalpath: you have everything right except for the last step...

(100g/58)/3.5L is the same as (100g/58) x (1/3.5L)--> because when you are changing the division to multiplication, you are simply flipping the denominator (which initially is 3.5/1 and gets flipped to 1/3.5) Thus, the denominator consists of 3.5 x 58, while the numerator is 100 x 1 [so 100/(3.5 x 58)]

 

[Lethstang]

jvs is correct, the answer is C

 

[DATlongshui]

I thought Kaplan did give answer E, not sure if the same question, but recall there is one such question in which their answer clearly say because no sol vol is given, so can't calculate.

 

[Questions]

then is it none of teh above??

 

[DATlongshui]

think so.

 

[jvss123]

the thing is for the very first question i posted, it was from their midterm test in which they said that the answer was not "none of the above." However, the second question i posted is from their lesson book where they say that you cannot calculate the molarity because you are given the volume of the solvent, not the solution. So, im still confused...