☰ Contents · Chemistry (General chemistry)

Solubility problems

Lessons 14 · 1 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
14

Solubility problems and their solutions

Textbook: pp. 70–73
GoalSolve problems with the solubility coefficient: finding S, the mass of solute or water needed to saturate, and the mass of substance that crystallises on cooling.
New words
saturating · to‘yintirishmass crystallised · kristallangan massamass of a saturated solution · to‘yingan eritma massasisolubility curve · eruvchanlik egri chizig‘i
Explanation

Solubility problems rest on the fact that in a saturated solution the ratio of the mass of solute to the mass of water is constant: m(solute)/m(water) = S/100. So 100 g of water and S g of salt make (100 + S) g of saturated solution and a proportion can be set up. To find S: S = m(solute)·100/m(water). The mass of solute needed to saturate is m = S·m(water)/100 and the mass of water needed is m(water) = m(solute)·100/S. The mass of solute to add to saturate a solution is the mass for saturation minus the mass already present. The mass that crystallises on cooling is S₁ – S₂ for every 100 g of water scaled to the given solution: first find the water and salt in the solution (S₁ g of salt per (100 + S₁) g of solution), then subtract the salt that stays dissolved at the lower temperature by its solubility. Likewise, when water is evaporated from a saturated solution m = S·m(evaporated water)/100 of salt separates.

Worked examples
At 40 °C 70 g of salt were needed to saturate 250 g of water: S = 70·100/250 = 28. So 28 g of salt dissolve in 100 g of water.
S(80 °C) = 80, S(20 °C) = 30. A 540 g saturated solution at 80 °C is cooled to 20 °C. 180 g of solution contains 100 g of water and 80 g of salt; 540 g contains 300 g of water and 240 g of salt. At 20 °C 90 g of salt stay dissolved in 300 g of water, so 240 – 90 = 150 g of salt crystallises.
Class activity

Worksheet: using a solubility table supplied by the teacher (KNO₃, NaCl, KCl), groups compare S at 20 °C and 60 °C and discuss which salt is easiest to purify by cooling.

Practice
1
If S = 25, how many grams of salt are needed to saturate 400 g of water?
2
If S = 28, in how many grams of water must 84 g of salt be dissolved to give a saturated solution?
3
216 g of an unsaturated solution contains 36 g of salt, and S = 30. How many more grams of salt must be added to saturate it?
4
S = 120 at 90 °C and S = 40 at 20 °C. A 330 g saturated solution at 90 °C is cooled to 20 °C. How many grams of salt crystallise?