Yttrium2
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Solution densities
Can someone explain solution densities?
How can I find the solution density?
How can I use a pipette to determine solution density, and why again are volumetric flasks more accurate ¿ than graduated cylinders?
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ELRIC
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Solution density is simply a ratio of the mass of the solution per its volume.
To ascertain the solution density, you simply need to be able to measure volume and mass; so you will need a balance to obtain the mass and a
graduated cylinder or a burette to obtain the volume. Solution density = mass divided by volume.
D=M/V
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Yttrium2
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How does weighing out as much solution as ones scale can handle
spread the error of the volumetric measurement out over a larger quantity?
It's not exactly intuitive.
[Edited on 7/11/2022 by Yttrium2]
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B(a)P
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It is all to do with the accuracy with which you can make your measurement vs the volume of your vessel.
Let's take a 100 mL volumetric flask. The neck is thin enough that you can measure the volume to plus or minus 0.1 mL. That gives you an accuracy of
plus or minus 0.1%.
Now a for a 100 mL measuring cylinder, the neck allows you to measure down to maybe 0.5 mL, probably more like 1 mL. That gives you an accuracy of 0.5
to 1% (assuming you are measuring the density of 100 mL of liquid). If you are using a 100 mL measuring cylinder to measure out 10 mL of liquid your
error goes up to 5 to 10%.
Now a for a 10 mL measuring cylinder, the neck allows you to measure down to 0.1 mL. That gives you an accuracy of 1% (assuming you are measuring the
density of 10 mL of liquid).
Hopefully this makes sense.
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CharlieA
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Belated New Year's resolution: Do not read any threads started by Yttrium2, or any of Yttrium2's replies/comments in any thread.
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Mateo_swe
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Nothing wrong with being new at chemistry or lacking chemistry education but still want to learn.
It is the "Beginnings" section after all, questions like this will surely come up.
One can use google but sometimes it doesnt explain things well.
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Yttrium2
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My question remains, why does weighing out more solute spread the error (standard deviation) of the measurement out more.
If I have enough solute to fill a 100mL graduated cylinder vs a 10mL graduated cylinder, wouldn't there be more error in calculating the density???
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Yttrium2
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The author of the quote says the more of the material placed on the scale, the more accurate the density measurement will be. -- The greater the mass
of material (H3PO3, in this instance) the greater the volume of the cylinder needed to make the measurement which should have more error, which is why
I'm asking this question.
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B(a)P
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The author of which quote?
You are correct in saying that as you move to larger vessels and larger scales* generally the accuracy of the piece of equipment is lower. I imagine
the assumption the 'author' is making is for the same piece of equipment the greater the volume and or mass the greater the accuracy for the reasons
stated in my post above.
*This is less true for scales because you can spend a small fortune and get very accurate scales with a large right range and high accuracy.
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Yttrium2
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I think your second sentence hits the nail on the head.
On second thought, why would a sample say 10 grams give a less accurate measurement of density than 30 grams?
My mind has been a little -- foggy lately, I think I'm going to take a break from here for a while.
Thanks for the response!
- Yttrium2 checking out.
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BromicAcid
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Assume your scale goes to 100.0 grams and reads to the tenths place. Also, assume the scale is actually accurate and has no significant intrinsic
error.
If you take a reading at 10.0 grams you don't know if it's 10.075 or 10.025 or something else, that last digit is up in the air, that's 1% of your
reading. But if you measure something out to 50.0 grams then your last digit is only 0.2% do you've decreased your uncertainty by 5x. Realistically
speaking through there are measurment / handling errors. By going to larger scales you help mitigate these. For chemicals that must be handled in
smaller quantities scales go beyond the tenths out to the hundred thousandths and beyond.
Remember though that density is not an intrinsic property of the solution. It varies by temperature.
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