Topscore Chem Question

[marymatthews]

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Question 64 on Topscore Test 3 for Chem asks:
If you dissolved 20 grams NaOH in 1000 grams H2O, then what is the molality of the solution?
Answer: 0.5
I understand you divide 20 g/40 g to get 0.5 moles NaOH.... then divide 0.5 moles/1 kg H2O to get 0.5.
But dont you also have to multiply by the van hoft factor of 2 in order to get 1.0? NaOH has a van hoft factor of 2? Isnt is ionic... which becomes (0.5)(2) = 1?
Thanks

 

[rpatel8]

When calculating molality, it is just mols solute/kg solution. The van hoft factor does not come into play. You only use this if you were trying to calculate freezing pt depression or B.P elevation, etc. Hope that helps

 

[gigawatt]

Good question. It's just that molarity and molality count formula units and not dissociated species by convention. If this question had asked for molarity (which has the same numerator), you wouldn't have ever thought to multiply by 2, so just get used to the same thing for molality.

 

[marymatthews]

But NaOH does have a van hoft factor of 2?
Heres my dilemma though....

Question 67 on Topscore Test 3 for Chem asks:
In order to reach equivalence point, what volume of a 5 M NaOH solution is needed to titrate a 0.25 L of a 4 M solution of HCl?
Answer: 0.2 L

Destroyer books claim that you must convert to NORMALITY when doing titrations.... thus, the formula is: NaVa = NbVb
Normaility of NaOH is 2
2 x 5 M x X L = 0.25 M x 4 L x 1
thus
X should equal 0.1 L

 

[UCB05]

But NaOH does have a van hoft factor of 2?
Heres my dilemma though....

Question 67 on Topscore Test 3 for Chem asks:
In order to reach equivalence point, what volume of a 5 M NaOH solution is needed to titrate a 0.25 L of a 4 M solution of HCl?
Answer: 0.2 L

Destroyer books claim that you must convert to NORMALITY when doing titrations.... thus, the formula is: NaVa = NbVb
Normaility of NaOH is 2
2 x 5 M x X L = 0.25 M x 4 L x 1
thus
X should equal 0.1 L

You have to understand that normality and the use of the van hoft factor is specific to whatever question you're looking at. When the question asks for the concentration of a solution, you should default to a factor of 1 (solute/solvent). If the question asked for concentration of ions, as in for colligative properties, then you would use a factor of 2 (particles/solvent). In a titration question, you look at the stoichiometric amount of acid or base it contributes; in your topscore example, it's 1 for NaOH, as it is for any other titration question (1 -OH per NaOH).

If Destroyer is asking the exact same question about titration with HCl and it's using a factor of 2, then it's wrong (it should be 1). However, since you said that Destroyer calls the normality of NaOH at 2N, I'm thinking you might have misread it, because normality already factors in the van hoft factor (N=iM), and there shouldn't be any further need to multiply by that factor again.

If you're just randomly trying to apply the van hoft factor to anything that ionizes, you should just ignore it entirely and just go by common sense. Approach concentration in normality by keeping that in mind.

 

[marymatthews]

ok that makes sense, thanks a lot
i have a side note question please...
how would you rate the following compounds in terms of strength and BP/MP?
Ionic, molecular (covalent), and network solid (covalent)
which of the three has the strongest bonds and the highest BP/MP? Which has the lowest?
thank you

 

[Ibraiz]

Ionic >>>>>> network solid > molecular