# mol calculation confusion - (Oct/08/2013 )

Hi friends,

I need to use KN-93 which is a CAMKII inhibitor. I ordered from sigma and is 0.1 mg! I cannot see anything in bottle and that's ok. It's in powder form. Although i used to feel comfortable with mol calculations, here i am totally confused.

Formula : C26H29ClN2O4S · H3PO4

Molecular Weight : 599,03 g/mol

I need a working concentration of 10 micromol/litre. Can anyone guide me on this?

After some calculation, i see that 1 mg= 1.67*10-6 mol i.e. 1.67*10-3 mM.

I need guidance after that. Need to know how much solvent i need to inject in this bottle to have 10 micromol/litre working conc.

thanks a lot,

K

First, figure out how many moles of your compound is in your vial:

(1e-4 grams)/(6e3 grams/mole) = 1.7e-8 moles

Next, figure out the amount of liquid to be added to make a 10 micromolar solution:

(1.7e-8 moles) / (x liters) = 10e-6 moles/liter

Rewriting,

x liter = (1.7e-8 moles) / (10e-6 moles/liter) = 1.7e-3 liter = 1.7 milliliters

This is high school algebra. You need to understand about carrying units and cancelling them along with your calculations.

You also need to understand why this statement is meaningless:

"After some calculation, i see that 1 mg= 1.67*10-6 mol i.e. 1.67*10-3 mM."

The units of moles (mol) and the units of moles/liter (mM) can never be equal.

Hi, thanks very much for the explanation . But why 6e3 grams/mole. Shouldn't it be 6e2 grams/mole ? the mol weight is 600 gm, not 6000. Or did i miss something?

So, i get

1.67e-7 moles in 0.1 mg powder.

Then i again didn't understand the part - (1.7e-8 moles) / (x liters) = 10e-6 moles/liter. could u please elaborate?

"After some calculation, i see that 1 mg= 1.67*10-6 mol i.e. 1.67*10-3 mM."

The units of moles (mol) and the units of moles/liter (mM) can never be equal.------------ point noted :)

You are correct, it should have been 6e2 g/mole.

So, i get

1.67e-7 moles in 0.1 mg powder.

This is correct.

What part of this don't you understand:

(1.7e-7 moles) / (x liters) = 10e-6 moles/liter.

I want to figure out how many liters (x) are needed to make a concentration of 10e-6 moles/liter.

I divide the number of moles I have (1.7e-7) by my unknown number of liters (x) and set it equal to 10e-6 moles/liter.

I solve for x by multiplying by x liter on both sides, and dividing by 10e-6 moles/liter.

I'm left with:

(1.7e-7 moles) / (10e-6 moles/liter) = x liters = 17e-3 liters = 17 ml

Note how the moles cancels and the 1/ (1/liter) becomes liter. It's helpful to explicitly carry units in your equations to get them right.

As an aside to the calculation conversation, I'm guessing your vial is probably not large enough to hold 17ml of solvent. I often create at 100x or 1000x stock from which to prepare my working solutions. If you were to make a 100x stock (1mM), you could add 170ul of solvent to your 0.1mg and then prepare a simple 1:100 dilution into your working solution when needed.

thanks both. It was really helpful. thank you very much.