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

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.

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

Molarity

Molality

Mole Fraction

% by mass

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

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.

Colligative Properties Lowering Vapor Pressure

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:

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-

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):