1. Lesson 6.1: Solutions and Concentration
Jeff Venables
Northwestern High School
Chemistry 1 Honors

2. Solution
Solute – the component(s) of a solution present in the smallest quantity.
Solvent – the component of a solution present in the greatest quantity.
Concentration – a measure of the relative amount of solute in a solution.

3. Concentration Solute and Solvent Concentrated vs. Dilute
Example – Sweet Tea
Concentrated vs. Dilute
Measurements of concentration
Molarity
Molality
Mole Fraction
% by mass

4. Molarity

5. Molality

6. Mole Fraction

7. % by mass

8. Example
17.85 g of NaCl is dissolved in 1000.g of water to make 1.04 L of solution. Calculate:
Molarity of NaCl
Molality of NaCl
Mole fraction of NaCl
% by mass of NaCl

9. Colligative Properties Lowering Vapor Pressure
Colligative properties depend only on quantity of solute molecules. (E.g. freezing point depression and boiling point elevation.)
Lowering Vapor Pressure
Non-volatile solvents reduce the ability of the surface solvent molecules to escape the liquid.
Therefore, vapor pressure is lowered.
The amount of vapor pressure lowering depends on the amount of solute.

10. Colligative Properties Lowering Vapor Pressure

11. Boiling-Point Elevation
A non-volatile solute lowers the vapor pressure of a liquid.
At 1 atm (normal boiling point of pure liquid) there is a lower vapor pressure of the solution. Therefore, a higher temperature is required to teach a vapor pressure of 1 atm for the solution (Tb).
Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, m:

13. i = vant Hoff factor
= number of particles produced when a substance dissolves
For nonionic substances, i=1
For ionic substances, i is the number of ions produced per formula unit that dissolves.
NaCl, i = 2 Na3PO4 i = 4
Na3PO4  3 Na+ + PO43-

14. Freezing Point Depression
The melting-point (freezing-point) curve is a near-vertical line from the triple point.
The solution freezes at a lower temperature (Tf) than the pure solvent.
Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant):