A mixture contains 100 g of potassium dichromate, K2Cr2O7, and 200 g of water. The mixture is saturated at 60 °C and then cooled to 20 °C, allowing crystals to form. What mass of K2Cr2O7 crystallizes from the solution during cooling?

• A. 9 g
• B. 18 g
• C. 31 g
• D. 62 g

Answer: (D)

Solubility depends on temperature, so a saturated solution at a higher temperature contains as much dissolved solute as the solvent can hold at that temperature. When you cool it, the solubility limit decreases and the excess solute crystallizes out until the solution is saturated again at the lower temperature.

For 200 g of water, the solubility of potassium dichromate at 60 °C is 50 g per 100 g water, so the solution can hold 100 g of K2Cr2O7 dissolved. Since the mixture is at saturation and contains 100 g, all of that is dissolved at 60 °C.

After cooling to 20 °C, the solubility is 19 g per 100 g water, so the solution can hold 38 g dissolved. Thus 100 g − 38 g = 62 g of K2Cr2O7 crystallizes out during cooling.