Preparing a Standard Solution
(a solution of known and accurate concentration)
One way is called the Primary Standard where a given solute is accurately weighed, dissolved and topped up to a certain volume in a volumetric flask.
But…this only works if your solute is pure and forms stable solutions.
- Solute (sodium carbonate)
- Wash bottle with de-ionised water
- Small beaker
- 100ml volumetric flask
- electronic scale
Equations: n = C x V m = n x M
Three Easy Steps…
1) Calculate mass of solute needed from amount of solute required, and weigh it in a small beaker.
- If 100ml of 0.20mol/L solution of sodium carbonate is required, amount of solute required is
n = CV = 0.20 x 0.1 = 0.02mol
- Hence, mass needed is:
m = nM = 0.02 x 106 = 2.12g
(since molar mass of Na2CO3 is 106g/mol)
- It’s a good idea to review these equations and have the calculations written up beforehand!
- Because it can be difficult to weigh the exact amount required, an amount close to this but accurately known can be used. In this case, take note of the final concentration, which will differ slightly from the intended.
2) Transfer the solute using a funnel and de-ionised water from a wash-bottle to the volumetric flask, taking care to get all the solute into the flask. Dissolve this solute with sufficient water in the flask.
3) Fill up the flask to the mark (allow for meniscus) with de-ionised water and VOILA! – a solution of known and accurate concentration!