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mol of solute vs mol of ions

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PagingDr.F

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(From AAMC FL1 C/P #58)
What volume of a 0.120 M CaI2 solution would contain 0.078 mol of SOLUTE?

Easy enough: 0.120 mol/L = 0.078 mol/xL
xL = 0.650 L

(Next-Step Science Diagnostic)
72g NH4Cl is dissolved in 1kg H2O. What is the molality of the resulting solution?

Molality = Moles of Solute / Mass of Solvent
72g NH4Cl = 1.35 mol >>> 1.35 mol NH4Cl / 1kg H2O = 1.35 m

BUT

NS is saying Moles of Solute = Moles of Ions:
1.35 Mol NH4CL >>> 1.35 mol NH4 + 1.35 mol Cl = 2.7 mol Solute
Molality = 2.7 mol/1 kg H2O = 2.7 m

When do we use Mol of Ions vs Mol of Solute? Both would ionize in solution so I'm confused...
 

PagingDr.F

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Next Step just emailed me back:

Hi there,

Thank you very much for reaching out! We typically do not reply to these emails (as they are for our own internal use in improving our materials), but I wanted to let you know that you have a good point here. The AAMC will typically specify "moles of ions" or refer directly to a colligative property like osmotic pressure if it is asking about the ions rather than the solute as a whole. If a question simply asks for molarity or molality, you typically do not need to worry about the ions a solute dissociates into.

We have updated the question to make what it is asking more clear. Thanks again for your feedback!

If anyone was interested... :yawn:
 

aldol16

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Good for them for fixing it. Chemists generally do not use the term "moles of solute" because it can be very confusing. What solute? When you're dissolving NaCl in water, you're solvating NaCl. But Na+ and Cl- are also solutes - they are what end up in solution. So chemists will almost always (good chemists will always do this) refer to "moles of NaCl" or "moles of Na+" or "moles of Cl-." We never use ambiguous terms if we can help it. We won't even use "moles of ions" like they say because that is also ambiguous. Are we talking about only cations or only anions? Or both together?
 
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