(Solved) Molality concentration PLUS density

biostudent123

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10 years ago

Lactose, C12H22O11, is a naturally occurring sugar found in mammalian milk. A 0.335M solution of lactose in water has a density of 1.0432 g/mL at 20∘C. What is the concentration of this solution in the following units?

a)Mole fraction: I got the anwser it is 6.46*10-3

b) Mass parcent: Got the answer it is 11.0 %

C)Molality: Got this one wrong twice, i dont know why, i used the equation moles of sulute over mass of solvent, but keep getting it wrong.

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Alexx

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10 years ago

Maybe you confused molarity with molality? But that's not very possible, because molarity is already given.
Here is my attempt:
Molality is: b=n/msolvent
msolvent is the mass of the solvent. Since the mass percentage of lactose is 11%, the solvent should be the rest 89%. This means that the solvent is: 0,89*m
Also, we know that c=n/V. And density is ρ=m/V <=> V=m/ρ=m/1,0432
So, we go back to c=n/V and replace: c=n/(m/1,0432) <=>c=1,0432*n/m <=> n=c*m/1,0432

Now, we can calculate molality: b=n/msolvent=(c*m/1,0432)/(0,89*m)=0,035/(0,89*1,0432)=0,0377mol/g (=30,77mol/kg if it's wanted in SI)

What did you find?

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biostudent123 Author

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10 years ago

this is what i found 0.361, I dont know how that answer came to be, maybe theres rounding involved or two step process of converting molarity to molality

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Alexx

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Molality concentration PLUS density

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