# Calculation help - (May/05/2018 )

Hi,

I have to make a solution and add it to the complete growth media (CGM) at 10 µM concentration.

The manufacturer says, to make the solution at 50 mg/ml working concentration. How can I calculate to get a concentration of 10 µM?

The final volume of the CGM with 10 µM concentration is 8 ml.

I think I can use C1V1 = C2V2, but I am confused with the units.

Thanks.

To wok out the molarity you need to know the molecular mass of the solute. If you know this you can substitute this into the equation N = mass/molar mass to give you number of mol per volume. (hint - you know there are 50 mg in 1 ml.., just make sure you convert this to g or convert the g/mol unit for your chemical to keep the units consistent!)

Now, as you know C*V also equals N, so you can use this to your advantage...

N = C * V

N= 10 uM * 8

N= 80 uM - this is the amount you want finally. From this you can also work

V= N/C

V= 80/ conc in uM of your solution.

bob1 on Sun May 6 02:15:30 2018 said:

To wok out the molarity you need to know the molecular mass of the solute. If you know this you can substitute this into the equation N = mass/molar mass to give you number of mol per volume. (hint - you know there are 50 mg in 1 ml.., just make sure you convert this to g or convert the g/mol unit for your chemical to keep the units consistent!)

Now, as you know C*V also equals N, so you can use this to your advantage...

N = C * V

N= 10 uM * 8

N= 80 uM - this is the amount you want finally. From this you can also work

V= N/C

V= 80/ conc in uM of your solution.

Thanks for the explanation Bob. The Mol weight of the chemical is 234 g/mol

When you say N = mass/molar mass, do you mean 234/50?

Other way around: 50/234 (or rather 0.05g/234 g/mol)...

bob1 on Sun May 6 02:41:18 2018 said:

Other way around: 50/234 (or rather 0.05g/234 g/mol)...

So, I calculated like this -

0.05 g/ 234 g/mol = 0.0002 mol (is the unit correct).

N = C x V = 80 µM

V = N/C = 80/0.0002 = 400,000 (What would be the unit here?)

Is the overall calculation correct now?

One more unit conversion - you have the top correctly in mol, but remember this is for a 50 mg/ml solution so that 0.0002 is actually 0.0002 mol/ml, The bottom is in micromol. Convert one or the other to get them to the same prefix - note that the 0.0002 is 0.2 mmol/ml and 80 umol is 0.08 mmol - if that helps keep it in some numbers that make sense. The final unit will be ml!

so V=N/C...

bob1 on Mon May 7 02:43:40 2018 said:

One more unit conversion - you have the top correctly in mol, but remember this is for a 50 mg/ml solution so that 0.0002 is actually 0.0002 mol/ml, The bottom is in micromol. Convert one or the other to get them to the same prefix - note that the 0.0002 is 0.2 mmol/ml and 80 umol is 0.08 mmol - if that helps keep it in some numbers that make sense. The final unit will be ml!

so V=N/C...

Thanks, now it makes sense. The final answer is 0.4 ml or 400 µl.