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Molarities, concentrations and ratios

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Molarities, concentrations and ratios

Postby bravebeaker » Thu May 12, 2011 1:00 am

Hi everyone,
I am a newbie on all this stuff and I need to do some calculations but I am out of adequate knowledge. I tried to search most of the forums here before posting, I apologize in advance if its a double post.

My question is if I need to add 50 uM of solution A to 50 ml of solution B and I need the ratio to be 1:1000 (A:B) how much volume do I need if the bottle of A has a label of 50 mM?

My second question is I need to use solution C which has a label of 3 uM in 500 ml. How much should I take to obtain 300 mM and add to solution B which is 50 ml?

I hope I addressed my questions in a clear way.

Looking forward to hearing from all of respected members..
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Postby protold » Thu May 12, 2011 5:18 am

Hola first question : diluting 1/1000 a 50mM solution you obtain a 50uM sol.. So if you have 50ml of B and have to add A 1/1000 to have a 50uMsol of A, you have to add 1000 times less of volume so 50ul is the answer. 50ml/1000=50ul.
Second one is less clair if sol C is 3 uM, is more diluted than 300mM so never you can get a 300mM sol starting of 3uM sol. But the inverse yes you can obtain a 3uM sol from a 300mM one. 300mM=300000uM so diluting 100000times you will get a 3 uM sol., so for 50ml you have to add 100000 times less volume that is the same that 0.5 ul. Hope that this helps you and don´t create more confusion. Buena suerte
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Postby JackBean » Thu May 12, 2011 6:56 am

let's try to make it a little easier. If you have change in units like from M to mM or mM to uM, that is 1000-times dilution, thus for every 1 ml you add 1 ul or for every 1 l you add 1 ml etc. I think this should make things easier.
Thus in the second case, if it was to make 3 uM solution from 300 mM one (since otherwise it makes no sense, as said protold already), you have to dilute it first from 300 mM to 3 mM, that is 100-times and then from 3 mM to 3 uM, which is 1000-times. Thus, if your finel volume is 50 ml, you add 1/1000, which is 50 ul, but then you need even 100-times less, thus 0.5 ul.

Hopefully that helps :)
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Postby bravebeaker » Fri May 13, 2011 9:31 am

Thank you both for your help! Now it make sense.
:D
AHH, Gravity! Such a cold cruel mistress.
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