Dilutions From Stock Solutions

Volumetric flasks used in diluting solutes with solvents
GIPhotoStock/Getty Images

If you're working in a chemistry lab, it's essential to know how to calculate a dilution. Here's a review of how to prepare a dilution from a stock solution.

Review Dilution, Concentration, and Stock Solutions

A dilution is a solution made by adding more solvent to a more concentrated solution (stock solution), which reduces the concentration of the solute. An example of a dilute solution is tap water, which is mostly water (solvent), with a small amount of dissolved minerals and gasses (solutes).

An example of a concentrated solution is 98% sulfuric acid (~18 M). The primary reason you start with a concentrated solution and then dilute it to make a dilution is that it's very difficult (sometimes impossible) to accurately measure solute to prepare a dilute solution, so there would be a large degree of error in the concentration value.

You use the law of conservation of mass to perform the calculation for the dilution:

MdilutionVdilution = MstockVstock

Dilution Example

As an example, say you need to prepare 50 ml of a 1.0 M solution from a 2.0 M stock solution. Your first step is to calculate the volume of stock solution that is required.

MdilutionVdilution = MstockVstock
(1.0 M)(50 ml) = (2.0 M)(x ml)
x = [(1.0 M)(50 ml)]/2.0 M
x = 25 ml of stock solution

So to make your solution, you pour 25 ml of stock solution into a 50 ml volumetric flask. Dilute with solvent to the 50 ml line.

Avoid This Common Dilution Mistake

It's a common mistake to add too much solvent when making the dilution.

Make sure you pour the concentrated solution into the flask and then dilute it to the volume mark. Do not, for example, mix 250 ml of concentrated solution with 1 L of solvent to make a 1-liter solution!