Question

Does solubility differ from molar solubility?

Answer 1

They differ only in the units used to measure them.

Solubility is the amount of solute that can dissolve in a given amount of solvent before the solution becomes saturated.

Scientists use many different units to express the solubility of a substance.

Usually, the solubility is listed as grams of solute per 100 g of solvent.

For example, the solubility of sodium chloride is 35.7 g/100 g of water at 20°C.

But you can use other units to express solubility such as g/100 mL and g/L.

Molar solubility is the number of moles of the solute that can dissolve per litre of solution before the solution becomes saturated.

EXAMPLE

The density of a saturated NaCl solution is 1.202 g/mL. What is the molar solubility of NaCl?

Solution

Assume that you have 35.7 g NaCl in 100 g of water. Then you have 135.7 g of solution.

Moles of NaCl = 35.7 g NaCl ×`(1"mol NaCl")/(58.44"g NaCl")` = 0.611 mol NaCl

Volume of solution = 135.7 g solution × `(1"mL soln")/(1.202"g soln")` = 112.9 mL

Molarity = `"moles"/"litres" = (0.611"mol")/(0.1129"L")` = 5.41 mol/L

Thus, the molar solubility of NaCl in water at 20 °C is 5.41 mol/L.