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# Concentration/molarity question (apologies)

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### #1 Peter Kullar

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Posted 11 October 2013 - 06:54 AM

Hi there,

Apologies for posting a basic questions.....

I have 25mg powder in my tube, mol weight = 258.23.  It needs to be reconstituted in water at min 7mg/ml.

I want to have a six well plate with 0.5mM, 1mM, 2mM and 5mM solutions in the wells.

By my calculation - need to dissolve 3.6ml water

Total moles in tube = 25*10-3 / 258.23 = 0.00096 moles

For 0.5mM well, need 0.5*10-3 * 2*10-3 = 1*10-6 moles (assuming that I put in 2 ml of media in the well)

Therefore I need 1*10-6/0.00096 = 0.001* 3.6 = 3.5 microlitre of the original solution in 2 ml of media in the well to make 0.5mM

is this right?

### #2 phage434

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Posted 11 October 2013 - 08:13 AM

This is a confusing way of solving this. I'd strongly suggest that you make a standard solution. Since you will be working with molar concentrations, you might make it at 1 M.

To make this solution, you need to add x liters of water to your compound, making a solution of concentration of [ 25 mg/(258 g/mole) ]/x liter = 1 mole/liter.

x liter = [ 25 mg/(258 g/mole) ] / 1 mole/liter = .000969 liter = 967 ul.

Now, you can calculate the amount of this solution you need in each well easily. For a well with 0.5 mM, you need to dilute the solution 2000x, so you if your well has a volume of 2 ml, you need 1 ul. For a well with 1 mM, you need to dilute by 1000x, so the volume you would add to 2 ml is 2 ul.

### #3 chandra3316

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Posted 11 October 2013 - 09:46 AM

### #4 chandra3316

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Posted 11 October 2013 - 09:59 AM

Please try to use the molarity calculator for your calculations. Do not make calculation complicated. the link is,

prepare stock for 20millimolar(25mg in 4.84ml)

5mM=500ul

1mM=100ul

0.5mM=50ul

of stock to final 2ml volume in the well. If you want to decrease above stock volumes make stock concentrted(100 milli molar or higher)

chandra3316@gmail.com

### #5 phage434

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Posted 11 October 2013 - 11:01 AM

If the only way you can solve a problem like this is to use a web based calculator, you have a problem.

Moreover, you won't be able to notice when the calculator is telling you to do something ridiculous.

Also, your solution above violates one of the constraints: that the stock solution be above 7 mg/ml.

### #6 Peter Kullar

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Posted 11 October 2013 - 10:32 PM

This is a confusing way of solving this. I'd strongly suggest that you make a standard solution. Since you will be working with molar concentrations, you might make it at 1 M.

To make this solution, you need to add x liters of water to your compound, making a solution of concentration of [ 25 mg/(258 g/mole) ]/x liter = 1 mole/liter.

x liter = [ 25 mg/(258 g/mole) ] / 1 mole/liter = .000969 liter = 967 ul.

Now, you can calculate the amount of this solution you need in each well easily. For a well with 0.5 mM, you need to dilute the solution 2000x, so you if your well has a volume of 2 ml, you need 1 ul. For a well with 1 mM, you need to dilute by 1000x, so the volume you would add to 2 ml is 2 ul.

Thanks a lot, yes, this is a much better way of working it out. thanks for your time.  best wishes, Peter