Water - Solutions and Solubility

If $25\text{ g}$ of Potassium Nitrate ($KNO_3$) is dissolved in $50\text{ g}$ of water at $313\text{ K}$ to form a saturated solution, what is its solubility at that temperature?

$\text{Solubility} = \frac{25\text{ g}}{50\text{ g}} \times 100 = 50\text{ g}$

Solubility is always calculated per $100\text{ g}$ of solvent. Since $50\text{ g}$ of water holds $25\text{ g}$ of solute, $100\text{ g}$ of water would hold $50\text{ g}$ of solute at the same temperature.

A solution contains $40\text{ g}$ of common salt ($NaCl$) in $320\text{ g}$ of water. Calculate the concentration in terms of mass by mass percentage of the solution.

$\text{Mass of Solution} = 40\text{ g} (solute) + 320\text{ g} (solvent) = 360\text{ g}$ $\text{Mass \%} = \frac{40}{360} \times 100 \approx 11.11\%$

To find the concentration, we first find the total mass of the solution by adding the solute and solvent, then divide the solute mass by the total mass and multiply by $100$.

What happens when blue crystals of $CuSO_4 \cdot 5H_2O$ are heated strongly in a test tube?

$CuSO_4 \cdot 5H_2O \xrightarrow{\Delta} CuSO_4 + 5H_2O \uparrow$

Upon heating, the blue hydrated copper sulphate loses its water of crystallization and turns into white anhydrous copper sulphate ($CuSO_4$). The water escapes as steam.