Lower secondary · 10 min

Preparing a solution of known concentration

Weigh copper(II) sulfate, dissolve it in water and see how the amount of solute controls the concentration.

Goal

Connect the microscopic act of dissolving to the definition $c_M = n/V$ and estimate the mass needed for a target molarity.

Apparatus and reagents

Copper(II) sulfate pentahydrate $\ce{CuSO4.5H2O}$ , distilled water, beaker, balance, graduated flask, stirring rod.

Procedure

Move the slider to a small mass of $\ce{CuSO4}$ and watch the crystals fall into the beaker of water.
Observe the particles leaving the solid and spreading until the blue colour is uniform — the solution is homogeneous.
Increase the slider: more solute in the same volume means a higher concentration $c_M = n/V$ .
Compute the mass needed for 250 mL of a 0.10 M $\ce{CuSO4}$ solution ( $M = 249.7\ \mathrm{g/mol}$ ) and check with the slider.

What to observe

The crystals disappear while the liquid takes an even light-blue tint — dissolving is complete.
Doubling the mass of solute at fixed volume doubles the concentration and deepens the colour.

Explanation

Dissolving disperses $\ce{Cu^{2+}}$ and $\ce{SO4^{2-}}$ ions among water molecules. The concentration $c_M = n/V$ counts moles of solute per litre of solution, so it rises with added mass and falls when the solution is diluted.

Chemists behind it

Amedeo Avogadro