Calculating solution concentration is something undergraduates can find confusing and frustrating. We’ve all been there when asked to make a solution of some compound to a specific concentration. We usually begin by collecting numbers from random places, then try fitting them together in the hope of coming up with a reasonable estimate. How much do I weigh out? What volume do I need? These pieces of the puzzle don’t always fall into place so easily, so just follow this simple procedure to do it right every time.
Step 1: Know What You Are Trying To Achieve
You can’t start until you know what you’re trying to achieve. The two essential pieces of information you need to get started are:
- The concentration you want your solution to be.
- The volume of solution you are trying to make.
Tip: Once you’re confident about the calculations, it’s useful to slightly overestimate how much volume you’ll need, to make sure you don’t run out.
Note: Solute is what is dissolved in the solvent (usually a solid). Solvent is the liquid that the solute is dissolved in, to form a solution.
Step 2: Determine How Many Moles You Need
This is simple to do once you know the concentration and volume of the solution you want. An important thing to note is that your concentration and volume are in the correct units.
- Concentration is in moles per Litre (mol/L).
- Volume is in Litres (L).
This doesn’t necessarily mean you need to make up a litre of solution – it just means that it’s in the format 0.1 L instead of 100 mL.
Once the units are sorted then you simply multiply the concentration by the volume (from Step 1) to produce the number of moles that you require.
- 1000 millilitres = 1 Litre
- 1000 milligrams = 1 gram
- No. of moles = concentration x volume
A good method of remembering this is to look at the units associated with concentration and volume (“x moles/L”: if you had 1L of a solution, then you’d have x moles present).
Step 3: Find The GFM Of Your Compound
The GFM (Gram Formula Mass) or molecular weight is the mass (in grams) of 1 mole of your compound. This magic number relates this calculation to YOUR compound.
It should have units of grams per mole (g/mol).
You can add up the atomic mass of each atom present in the chemical structure of your compound and that will give you a value for the GFM (e.g. mass of CH4 = 12 + 1 x 4 = 16 g/mol).
Tip: This value is normally listed on the bottle of the compound, or can be easily found on the internet (Sigma-Aldrich, etc.)
Step 4: Use The GFM To Determine The Mass of Compound Required
Multiply the number of moles you’ll need of your compound (from Step 2) by the GFM of your compound (Step 3).
Tip: Helpful ways to remember this calculation are:
- Use this helpful triangle :
2. Consider the units:
GFM is in g/mol and you’re working out how many grams a certain number of moles will be. Therefore multiply GFM by the no. of moles
Step 5: Weigh out your Compound
This is done in the laboratory using an accurate balance, and involves weighing out the correct weight of your compound onto a weighing boat or watch-glass tared to zero on the scales.
Tip: Good laboratory skills help here:
- Don’t ever put any excess compound back into the bottle
- Don’t be afraid to use a smaller spatula for greater accuracy
Step 6: Into Solution
Now it’s time to make your weighed out compound (the solute) into a solution by adding it to the solvent. This is done using the correct sized volumetric flask for the volume you want.
You can either add the solid to the flask directly or sometimes it’s easier to dissolve it first (in a small volume of the solvent) before pouring it into the volumetric flask.
Once this is done you need to carefully make the level of the solution up to the mark on the neck of the volumetric flask.
Be careful not to overshoot as this will cause a large error in the accuracy of the concentration of your solution! (Use a pipette for those last few centimetres).
Step 7: Mix Well
Once the correct volume has been achieved the only thing to do is to make sure you mix the solution well! Always invert the flask a few times before using, and make sure you have a finger on the stopper while you mix.
You have successfully made a solution to the correct concentration!