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Offline baffled9001

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concentrations hwk question
« on: May 23, 2020, 02:50:32 AM »
Hi everyone, I got stuck on a homework problem and was hoping for some help.

It's "explain why this experimental procedure is incorrect: 'To make 1.00 L of a 1.00 M NaCl solution, I will dissolve 58.5 g of NaCl in 1.00 L of water.' "

But 1 L solution * 1 mol NaCl/1 L solution * 58.44 g NaCl/1 mol NaCl = 58.44 g NaCl. So wouldn't dissolving 58.44 g NaCl in 1 L H2O result in a 1 M NaCl solution, after all? I'm not seeing how it doesn't.

So then I thought the problem might have more to do with experimental procedure than with calculations. But when I tried looking up "how to prepare a 1 M solution of NaCl" on YouTube, the two people I watched did exactly that. They poured most of 1 L of water into a container, poured the 58.44 g NaCl in, poured the remaining water up to the 1 L mark (being careful not to go over), and mixed it until it dissolved.

I mean, technically you'd be using slightly less than 1 L of water, because the container's containing the NaCl, too. But somehow, I don't think that's what the problem's asking.

And sure, the molar mass I got for NaCl (58.44 g) isn't exactly the same as the 58.5 stated in the problem, but I don't think that's it, either. If it was a significant figures question, I'd understand, but this is part of a Concentrations lab.

Am I missing something here?

Offline AWK

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Re: concentrations hwk question
« Reply #1 on: May 23, 2020, 03:34:28 AM »
Am I missing something here?
You have not read the manual carefully.

There are two types of concentration: molar and molar, often confused because of the similarity in the name. In addition, for very low concentrations, the values of these concentrations are almost the same.
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Offline Borek

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Re: concentrations hwk question
« Reply #2 on: May 23, 2020, 03:59:43 AM »
molar and molar

Molar and molal :)

So wouldn't dissolving 58.44 g NaCl in 1 L H2O result in a 1 M NaCl solution, after all? I'm not seeing how it doesn't.

Hint: the density of 1 M solution is 1.0378 g/mL.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline AWK

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Re: concentrations hwk question
« Reply #3 on: May 23, 2020, 04:07:26 AM »
molar and molar
Molar and molal :)
Unnoticed printing error - sometimes it happens, but it should happen as rarely as possible.
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Offline mjc123

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Re: concentrations hwk question
« Reply #4 on: May 23, 2020, 06:06:43 PM »
Quote
I mean, technically you'd be using slightly less than 1 L of water, because the container's containing the NaCl, too. But somehow, I don't think that's what the problem's asking.
That is exactly what the problem's asking. That's why you make up a solution as they did on YouTube.
Note that a 1M solution is defined as 1 mole of solute in 1 L of solution (not 1 L of solvent).
Using Borek's density figure, 1 L of solution would contain 1038 g, of which 58 g is NaCl, so it only contains 980 g water.

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