Homework Help: Chemistry

Posted by S on Tuesday, January 24, 2012 at 4:08pm.

An aqueous solution is 8.50% ammonium chloride, NH4Cl, by mass. The density of the solution is 1.024g/mL. Calculate the molality, mole fraction, and molarity of NH4Cl in the solution.

I know the formulas needed but I'm not sure where to start.

M = moles solute/1 L solution

m = moles solute/1 kg solvent

x = moles solute/total moles of solution

Can I have some help please with where to get started on this problem? :) Thanks!

Chemistry - DrBob222, Tuesday, January 24, 2012 at 4:17pm
I would start with 8.50% NH4Cl which you know means 8.50 g NH4Cl per 100 g soln.
You know the density so you can convert 100 g soln to volume (mL) and convert 8.50 g NH4Cl to moles. That gives you an easy M = moles/L soln.
Next I would look at the % soln again. You know that is 8.50 g NH4Cl/(8.50g NH4Cl + 91.50g H2O). You know moles from the earlier step so this give you an easy m = moles/kg solvent.
Now that you know moles NH4Cl and kg solvent, you can convert to moles solvent and get X from that for NH4Cl (and X for H2O if you want it).
Chemistry - S, Tuesday, January 24, 2012 at 9:58pm
Thanks so much for the feedback! I think I figured the answers out! :)

Molarity = 0.161 M

Molality = 1.74 m

Mole Fraction = 1.57E-4 moles NH4Cl
Chemistry - S, Tuesday, January 24, 2012 at 10:18pm
Made a mistake.

****Mole Fraction = 0.0303 moles NH4CL
Chemistry - DrBob222, Tuesday, January 24, 2012 at 11:27pm
I obtained 1.627 which rounds to 1.63 for M. I used 53.49 for the molar mass NH4Cl.
I agree with the other two answers.

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