chemistry/math help needed(stock solutions)

[dln1128]

This is the problem. I have a stock solution of a chemical which is 10mg/ml
I have to make agar plates that contain a a range of concentrations from 5ug/ml to 50ug/ml. Could someone help me figure out how to solve these types of problems.

 

[tkhunny]

They are all the same. For your general application, you can create a general solution.

s = Amount of Desired solution
c = Concentration of Desired Solution

The trick is to equate something. In this case, it will be a lot easier to equate "a chemical", since we'll be adding a solution that is 0 mg/ml - water?

There are three elements:

1) Amount of Original Solution*Concentration of Original Solution = Amount of "a chemical"

2) Amount of Dilution Compound * 0 = 0

3) Amount of Desired Solution*Concentration of Desired Solution = s*c = Amount of "A Chemical"

1 + 2 = 3

You should see that the chemical AMOUNT is the same on both sides of the equation. This gives

Amount of Original Solution*Concentration of Original Solution = s*c

Solve for the one thing we don't know.

Amount of Original Solution = [s*c]/[Concentration of Original Solution]

We know that: Concentration of Original Solution = 10 mg/ml

Amount of Original Solution = [s*c]/[10 mg/ml]

So, if we want 25 ml of 5 ug/ml solution

Amount of Original Solution = [(25 ml)*(5 ug/ml)]/[10 mg/ml] = 0.0125 ml

So, if we want 75 ml of 50 ug/ml solution

Amount of Original Solution = [(75 ml)*(50 ug/ml)]/[10 mg/ml] = 0.0375 ml

I hope you're not doing anything dangerous. The arithmetic is one thing, but caution in the lab is quite another. :shock: