Problem I:
Calculate the molality and mol fraction
of HCl for a solution which has a mass percent of 37.1 %.
Answer:
This problem is a little different compared
to problems we've done in class. I did not tell you how much of the solute and
the solvent is available, I just gave you the percentage by weight of the solute
HCl.
But that is OK we can assume any amount
of solute and solvent and still get the correct answer.
So you assume an amount of solute and solvent,
but just remember is must be 37.1% by weight HCl.
Did you come up with 37.1 grams of HCl and 62.9 grams of H2O?
This is how the expert thinks....If the solution is 37.1% by
weight HCl according to the definition of weight % that means in 100 grams
of solution there must be 37.1 grams of HCl. If there is 100 grams of solution
and only HCl and H2O are present than 100 grams - 37.1 gram = 62.9
grams H2O.
We were not given an amount of HCl or H2O so we can
make up how much we want! The easiest way is to assume an amount in terms of
the weight % definition. Click here for a quick text
of this logic
Now we can calculate the molality of HCl in the solution.
What is the definition of molality?