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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: warprin on September 03, 2009, 12:13:41 PM

Title: Molality from molarity of unknown solute
Post by: warprin on September 03, 2009, 12:13:41 PM
I have been looking at this for 2 hours and I'm missing something very basic.

I have three solutions: 25mL of A, 50mL of B, 100 mL of C. They all have the same solution density, which is unknown. All contain 0.041 moles of ions (aqueous ionic solution). However, I don't know what these ions are, but each solution has a particular number of ions of various charges and sizes drawn in the solutions: A has 8 ions, B has 10 ions and C has 12 ions. I calculated molarity for each at 1.64, .82 and .41 since I know the number of moles and the mass of the solution.

In trying to calculate molalities, I need the moles of solute (which I know) and the mass of the solvent, and it's the latter I'm stuck on.

Would someone point me in the right direction? I just don't see how I can calculate the mass of the solvent (even if I assume it's highly diluted and use molarity instead).

Thanks!
Title: Re: Molality from molarity of unknown solute
Post by: MrTeo on September 04, 2009, 06:59:05 AM
Here's something that could be quite useful to you: M stands for molarity, m is the molality (msolvent and msolute are the masses), Mr is the molecular mass (relative) and (https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.forkosh.dreamhost.com%2Fmimetex.cgi%3F%7B+%5Cdelta+%7D&hash=9da9e34d55ddff3ed9e9569cfa1fe6206bea80b6) the density.

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fwww.forkosh.dreamhost.com%2Fmimetex.cgi%3F%7B+%5Cbegin%7Barray%7D%7Bl%7D%3Cbr+%2F%3Em+%3D+%5Cfrac%7Bn%7D%7B%7B%5Cfrac%7B%7Bm_%7Bsolvent%7D+%7D%7D%7B%7B1000%7D%7D%7D%7D%5C%5C+%3Cbr+%2F%3E%5C%5C%3Cbr+%2F%3Em+%3D+%5Cfrac%7B%7BM+%5Ccdot+V_%7Bsolution%7D%7D%7D%7B%7B%5Cfrac%7B%7Bm_%7Bsolution%7D-m_%7Bsolute%7D%7D%7D%7B%7B1000%7D%7D%7D%7D%5C%5C+%3Cbr+%2F%3E%5C%5C%3Cbr+%2F%3Em+%3D+%5Cfrac%7B%7BM+%5Ccdot+V_%7Bsolution%7D%7D%7D%7B%7B%5Cfrac%7B%7B%5Cdelta+_%7Bsolution%7D%5Ccdot+V_%7Bsolution%7D-+n+%5Ccdot+M_r+%7D%7D%7B%7B1000%7D%7D%7D%7D%5C%5C+%3Cbr+%2F%3E%5C%5C%3Cbr+%2F%3Em+%3D+%5Cfrac%7B%7BM+%5Ccdot+V_%7Bsolution%7D%7D%7D%7B%7B%5Cfrac%7B%7B%5Cdelta+_%7Bsolution%7D%5Ccdot+V_%7Bsolution%7D-M+%5Ccdot+V_%7Bsolution%7D+%5Ccdot+M_r+%7D%7D%7B%7B1000%7D%7D%7D%7D%5C%5C+%3Cbr+%2F%3E%5C%5C%3Cbr+%2F%3Em+%3D+%5Cfrac%7B%7BM+%5Ccdot+V_%7Bsolution%7D%7D%7D%7B%7B%5Cdelta+_%7Bsolution%7D-%5Cfrac%7B%7BM+%5Ccdot+M_r+%7D%7D%7B%7B1000%7D%7D%7D%7D%5C%5C+%3Cbr+%2F%3E%5Cend%7Barray%7D+%7D&hash=37ef3bf26b9f00f82ccd28964fa814a62ab1a9fb)

The units used are g/L for the density (even if dividing it by 1000 in the last passage we convert it to g/cm3), grams and liters for volume.
Title: Re: Molality from molarity of unknown solute
Post by: Borek on September 04, 2009, 07:15:06 AM
Printable, downloadable concentration cheat sheet (http://www.chembuddy.com/?left=CASC&right=concentration-cheat-sheet).

Mass of solvent - assuming diluted solutions it is just volume times density (1 g/mL for water).