How do I dilute to make a stock solution, chem lab final?
So my chem lab final is coming up, and this is what the email said it entails:
The practical will test your ability to make a stock solution of a specified concentration from a solid and then make a dilution of specified concentration from the stock solution.
Can someone please explain what this means in less general terms, and perhaps do an example or give a useful link? Maybe you’ve even done this before!
I’m worried about this and I really need an A, so anything to help me better understand this would be great!
Thanks!
A stock solution is a fairly concentrated solution from which, by dilution, you can easily make solutions of different concentrations.
So adding an x volume of stock to a y volume of water, rather than weighing out the solute for each solution. So one-time weighing only.
Example?
you have specified concentration. Let’s say X M (of a substance with mol mass of MolMass)
You make specific volume, say v L
then you weigh out (v/1000) * X * MolMass and disssolve in v liter
Now you need a solution of 0.25 X
That’s 1/4 of the stock, so take a volume of stock and add 3 times that volume in water. (eg, 20 ml stock plus 60 ml water)
Now you need a solution of 0.1 X
That’s 01/10 of the stock concentration, so take a volume of stock and add 9 times that volume in water (eg 5 ml stock plus 45 ml water)
And so on.
NOTE you can only go DOWN in concentration, not up!!
And yes, I have done this myself. I worked in an anatomical lab and we used rather a lot of phosphate buffer. Once a week one of us made a 1M stock solution which then during the week, was diluted by the various lab workers to suit their particular needs. Since we used a lot, and there were 8 of us, we made 2000 ml stock (just to be on the safe side). Making the stock was not the most interesting job, so it rotated among the workers. Getting it right would take careful weighing and dissolving and given the quantities would easily take 30 to 45 minutes. But it sure paid off during the week!
Good luck on your lab.