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#1
05-15-2012, 09:08 AM
 Unregistered Guest Posts: n/a
w/v solution vs w/w solution

I have a 50% citric acid solution (w/v) that i prepared to speed up my process vs weighing out each time I perform test. My question is how can I calculate how much I am actually adding when I pipette 2mLs. I believe I am not adding the same amount as if I were to weigh it out each time. Also, is there much difference if I made a solution on a w/w basis? Thanks
#2
05-15-2012, 10:19 AM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,234 Rep Power: 18
Re: w/v solution vs w/w solution

Quote:
 Originally Posted by Unregistered I have a 50% citric acid solution (w/v) that i prepared to speed up my process vs weighing out each time I perform test. My question is how can I calculate how much I am actually adding when I pipette 2mLs. I believe I am not adding the same amount as if I were to weigh it out each time. Also, is there much difference if I made a solution on a w/w basis? Thanks
I presume 50% means 0.5 g/mL. So if you pipette 2 mL, you are adding 1 g of citric acid. That assumes your lab is temperature controlled and the volume of the solution is not changed with temperature swings.

If you make a solution on a w/w basis, you either have to weigh out the solution or know the density to switch to a volume basis. If you made a 50 % w/w solution, that would be 0.5 g/g and you'd have to weigh out 2 g of solution for 1 g of citric acid.
#3
05-15-2012, 11:17 AM
 Mrs X can't count, can't spell! Join Date: Feb 2006 Location: New Zealand Posts: 2,383 Rep Power: 11
Re: w/v solution vs w/w solution

OP, are you asking how much 2mL of the 50% citric acid solution weighs?

I'm a chemist, and for most of us it would be much quicker pipetting something than weighing it. You can always do a timed test, pipette out 10 lots of 2mL, and weigh out 10 lots of whatever the weight is.

To work out how much 2mL weighs, just pipette the 2mL into a tared vessel in a balance.

Last edited by Mrs X; 05-15-2012 at 04:11 PM. Reason: clarity. hopefully!
#4
05-16-2012, 04:19 AM
 Unregistered Guest Posts: n/a
Re: w/v solution vs w/w solution

Thank you for replies. My concern is that I am not adding the same amount if I pipette versus weighing out the crystals. I thought that when you added amount by volume (pipette) I needed to consider density. When I first made the solution (50 grams citric acid and fill to 100mL mark in volumetric) I did not think about density but now as I am trying to work through the math, I am confused as to whether this 50%w/v solution is actually delivering the required amount. I am adding 3mLs of 50%w/v citric acid where test calls for adding 1.5 grams. So I am still confused if the 3 mls is delivering 1.5 grams? I also do not understand how much I would be adding if I used a 50%w/w citric acid solution. I would really appreciate if you could help me get my head around these calculations and if i need to include density.
#5
05-16-2012, 10:59 AM
 Mrs X can't count, can't spell! Join Date: Feb 2006 Location: New Zealand Posts: 2,383 Rep Power: 11
Re: w/v solution vs w/w solution

Quote:
 Originally Posted by Unregistered Thank you for replies. My concern is that I am not adding the same amount if I pipette versus weighing out the crystals. I thought that when you added amount by volume (pipette) I needed to consider density. When I first made the solution (50 grams citric acid and fill to 100mL mark in volumetric) I did not think about density but now as I am trying to work through the math, I am confused as to whether this 50%w/v solution is actually delivering the required amount. I am adding 3mLs of 50%w/v citric acid where test calls for adding 1.5 grams. So I am still confused if the 3 mls is delivering 1.5 grams? I also do not understand how much I would be adding if I used a 50%w/w citric acid solution. I would really appreciate if you could help me get my head around these calculations and if i need to include density.
If 100mL contains 50g citric acid, then 3mL contains 1.5g citric acid, as your concentration is always going to be 50% w/v.

Just for argument, if you had made up your solution with 50g of Citric acid, and added 50g of water, you wouldn't have 100mL, and you would have to weigh out 3g of solution to get the desired 1.5g in your reaction.

(I'd do it the way you are doing it as well. - All lab glass ware is designed to be volumetric, which is why traditionally solutions are always made up w/v.)
#6
05-16-2012, 12:10 PM
 Unregistered Guest Posts: n/a
Re: w/v solution vs w/w solution

Thanks, I guess I cannot get past the fact of not using density. My other issue I am hoping you can help me with is about the preparation of the 50% solution. I weighed out the citric acid and then brought it to the 100mL mark with water. When I mixed the solution, the level decreased aproximately 10 mls. So I am do not know if I should top this up with additional water or not.
#7
05-16-2012, 02:30 PM
 JohnS Double Ultimate Supreme Member Join Date: Dec 2007 Location: SE Michigan, USA Posts: 9,234 Rep Power: 18
Re: w/v solution vs w/w solution

Quote:
 Originally Posted by Unregistered Thanks, I guess I cannot get past the fact of not using density. My other issue I am hoping you can help me with is about the preparation of the 50% solution. I weighed out the citric acid and then brought it to the 100mL mark with water. When I mixed the solution, the level decreased aproximately 10 mls. So I am do not know if I should top this up with additional water or not.
You need density if you try to measure volume of w/w solution, or weigh a w/v solution; don't cross systems.

If you mix to a w/v, the easy way is to measure volume. If you mix a w/w solution, weigh the reactant.

Be sure to top it up to 100 mL. I don't think it is a concern with citric acid, but some mixtures will heat up or cool off when mixed. You want to achieve the desired volume when it returns to the standard temperature of your lab. With thermal expansion and temperature changes, the concentration of your w/v solution changes (slightly).
#8
05-16-2012, 09:57 PM
 Mrs X can't count, can't spell! Join Date: Feb 2006 Location: New Zealand Posts: 2,383 Rep Power: 11
Re: w/v solution vs w/w solution

Quote:
 Originally Posted by Unregistered Thanks, I guess I cannot get past the fact of not using density. My other issue I am hoping you can help me with is about the preparation of the 50% solution. I weighed out the citric acid and then brought it to the 100mL mark with water. When I mixed the solution, the level decreased aproximately 10 mls. So I am do not know if I should top this up with additional water or not.

Instead of density, you are using concentration (50g/100mL). Probably you need to go to a couple more lab classes to understand it better, but hopefully it will make sense after you have been using these solutions for a while.

To accurately make up a solution of 50g of critic acid in 100mL of solution, you need to weigh out 50g into your 100mL volumetric flask, make sure you wash in any extra from your weighing vessel, add enough water to dissolve and thoroughly mix the citric acid, bring it to room temperature, and then add enough water with mixing to bring it up to the mark. There would be no point even using the volumetric flask if you don't make the solution up to the mark. Hope this makes sense, you are welcome to keep asking questions in the mean time.

Your solution will have changed volume while is was dissolving, you HAVE to make up to the mark after all the crystals have dissolved.
#9
05-17-2012, 07:48 AM
 Unregistered Guest Posts: n/a
Re: w/v solution vs w/w solution

Yep, I need a lot more than just a few lab classes. lol not a chemist at heart but in a chemists job. Can I painfully ask one more question that I am pretty sure I already know the answer to. If I inject 1uL of each of these solutions into an HPLC I will need to account for density on the w/w solution by multiplying by density to accurately compare the w/w to w/v solutions...correct?
#10
05-17-2012, 10:32 PM
 Mrs X can't count, can't spell! Join Date: Feb 2006 Location: New Zealand Posts: 2,383 Rep Power: 11
Re: w/v solution vs w/w solution

Quote:
 Originally Posted by Unregistered Yep, I need a lot more than just a few lab classes. lol not a chemist at heart but in a chemists job. Can I painfully ask one more question that I am pretty sure I already know the answer to. If I inject 1uL of each of these solutions into an HPLC I will need to account for density on the w/w solution by multiplying by density to accurately compare the w/w to w/v solutions...correct?
lol, if you are injecting these into an HPLC, you are seriously endangering your life if you aren't a chemist at heart, and you haven't talked it through THOROUGHLY with the person who looks after the HPLC.

(HPLC mixtures need to be very pure, and it takes a fair bit of chemistry skill to get the solutions clean enough. I have had the HPLC technician crying and swearing on my shoulder because the person putting stuff through the system didn't talk it through with them first. Said tech would then get blamed when they had to take a week cleaning the HPLC).

Any chromatography i've done just uses known weights (You know the concentration, so you know the weight of the material of interest you are injecting), and compares them to a standard.

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