CHEMISTRY
posted by lulu78 on .
brass is a subtitutional alloy consisting of a solution of copper and zinc. a particulae sample of red brass consisting of 80% copper and 20% zinc by mass has a density of 8750kg/m^3.
a. what is the molality of zinc in the solid solution and
B. what is the molarity of zinc in the solution

20% Zn means 20 g Zn/100 g soln.
Convert 20 g Zn to moles. moles = grams/molar mass = ??
molality = ??moles/0.1 kg = xx m
Use density to convert 0.1 kg soln to volume (L), then M = moles/L 
i don't understand why i cant get this.
I have the answers but i don't get them.
i did 20g/65.4g/mol= 0.306mol,
then 0.306mol/0.1kg= 3.058m, but the answer they have is 3.82m.
then the next step still confuses me. 
I don't see anything wrong with 3.06m.
For M, convert 100 g to volume.
You have 0.306 moles/100 g
volume = mass/density = 100g/8.75 = 11.14 cc = 0.01114 L and
0.306moles/0.01114 L = ??M 
dumb question?
why is it 100g not 0.1kg?
the rest i get/got. 
Hang on. I know what I did wrong and it was a STUPID mistake. I thought of my mistake while taking a shower. Give me a minute to dry off and I will correct it for you.

OK. Am I embarrassed. Big time.
20% Zn means 20 g Zn/100 g soln or
20 g Zn/(20g Zn+ 80g Cu).
molality = moles/kg SOLVENT (not soln).
20 g is 0.306 moles and that in 0.08 kg Cu(as the solvent) = 3.82m
Perhaps I had better look at the molarity one also. 
so the rest is good then?

I don't think I divided right on the calculator for molarity.
100 g soln/8.75 = 11.43 cc (not the 11.14 I typed. That was a typo.
Then 0.306 moles/0.01143 = 26.8 M but you should confirm all of these calculations. I'm punch drunk==punching the wrong numbers on the key pads. 
i have another question on this silly topic further up if your fresh and ready again.

I'm not fresh (I've been at this all day, meaning about 12 hours) but post away. I'm on the way to bed but I'll look at your question.

how many kg of Zn in 3.60 kg of ZnS