Homework Help: CHEMISTRY

Posted by lulu78 on Thursday, October 14, 2010 at 11:10pm.

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

CHEMISTRY - DrBob222, Thursday, October 14, 2010 at 11:27pm
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
CHEMISTRY - lulu78, Thursday, October 14, 2010 at 11:46pm
i dont understand why i cant get this.
I have the answers but i dont 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.
CHEMISTRY - DrBob222, Friday, October 15, 2010 at 12:02am
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
CHEMISTRY - lulu78, Friday, October 15, 2010 at 12:10am
dumb question?
why is it 100g not 0.1kg?
the rest i get/got.
CHEMISTRY - DrBob222, Friday, October 15, 2010 at 12:23am
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.
CHEMISTRY - DrBob222, Friday, October 15, 2010 at 12:29am
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.
CHEMISTRY - lulu78, Friday, October 15, 2010 at 12:40am
so the rest is good then?
CHEMISTRY - DrBob222, Friday, October 15, 2010 at 12:43am
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.
CHEMISTRY - lulu78, Friday, October 15, 2010 at 12:44am
i have another question on this silly topic further up if your fresh and ready again.
CHEMISTRY - DrBob222, Friday, October 15, 2010 at 12:46am
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.
CHEMISTRY - yyy, Saturday, January 22, 2011 at 2:43pm
how many kg of Zn in 3.60 kg of ZnS

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