Lessons 16–17 · 2 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
16
Percent concentration problems and their solutions
Textbook: pp. 77–84
GoalSolve harder percent-concentration problems: substances that react with water, mixing and diluting solutions, and gas evolution.
New words
mixing solutions · eritmalarni aralashtirishdilution · suyultirishreaction with water · suv bilan reaksiyamass balance · massa balansi
Explanation
If a substance reacts with water (Na, Na₂O, K₂O, SO₃, SO₂ and others), the concentration of the resulting solution is calculated from the mass of the new substance formed (NaOH, H₂SO₄ …) using the reaction equation, not from the original substance. If a gas is released, the mass of the solution is m₂ = (sum of the starting masses) – m(gas): in 2Na + 2H₂O → 2NaOH + H₂↑ the mass of hydrogen is subtracted. When solutions are mixed, the masses of solute add and the masses of the solutions add: C% = (m₁′ + m₁″)/(m₂′ + m₂″)·100 %. On dilution the mass of solute is unchanged: m₂′·C%′ = m₂″·C%″, and the mass of water added is m₂″ – m₂′. In every problem: 1) write the reaction equation, 2) find the mass of solute, 3) find the mass of the solution (allowing for any gas), 4) calculate C%.
Worked examples
7 g of lithium (1 mol) is added to 114 g of water: 2Li + 2H₂O → 2LiOH + H₂↑. LiOH is 1 mol = 24 g; H₂ is 0.5 mol = 1 g. m₂ = 7 + 114 – 1 = 120 g; C% = 24/120·100 % = 20 %.
150 g of 12 % solution and 250 g of 28 % solution are mixed. Salt: 18 + 70 = 88 g; solution: 400 g; C% = 88/400·100 % = 22 %.
Class activity
Group contest: each group presents on the board the four-step scheme «reaction – mass of solute – mass of solution – C%» for its own solution problem.
Practice
1
80 g of SO₃ is added to 120 g of water (SO₃ + H₂O → H₂SO₄). Find the mass fraction (%) of H₂SO₄ (M(SO₃) = 80, M(H₂SO₄) = 98).
49
2
How many grams of water must be added to 200 g of a 30 % solution to make it 20 %?
100
3
150 g of 8 % solution and 50 g of 24 % solution are mixed. What percent is the new solution?
12
4
Why is the result not always 20 % when a 10 % and a 30 % solution are mixed?
The percentages refer to different masses; the result depends on the masses of the solutions, so the masses of solute and of solution are added separately. Only for equal masses is it 20 %.