How To Calculate Solubilities

Solubility is measured either in grams per 100 g of solvent – g/100 g – or number of moles per 1 L of the solution. As an example, calculate the solubility of sodium nitrate, NaNO₃, if 21.9 g of the salt is dissolved in 25 g of water. Based on this calculation, the final volume of the NaNO₃ saturated solution is 55 ml. Solubility indicates the maximum amount of a substance that can be dissolved in a solvent at a given temperature. Such a solution is called saturated.

Step 1

Divide the mass of the compound by the mass of the solvent and then multiply by 100 g to calculate the solubility in g/100g . Solubility of NaNO₃=21.9g or NaNO₃ x 100 g/ 25 g =87.6.

Step 2

Calculate the molar mass of the dissolved compound as the sum of mass of all atoms in the molecule. Atomic weights of corresponding elements are given in the periodic table of the chemical elements. (See Resources). In the example, it would be: Molar mass NaNO₃=M(Na)+M(N)+3 x M(O)=23+14+3x16=85 g/mole.

Step 3

Divide the mass of the dissolved compound by its molar mass to calculate the number of moles. In our example, this would be: Number of moles (NaNO₃)=21.9g /85 g/mole= 0.258 moles.

Step 4

Divide the number of moles by the solution volume in liters to calculate solubility in mole/L. In our example, the solution volume is 55 mL or 0.055 L. The solubility of NaNO3=0.258 moles/0.055 L=4.69 mole/L.