Topic: Solutions Aim: What is another way to calculate concentration? DN: What is the formula for molarity?

MOLALITY Another way of expressing solution concentration Molality (m) = moles of solute/kg of solvent k50 (watch to 1:45) k50 Demo w/1000mL grad cylinders and stoppers

Why/when would we use this? Youtube video (what’s the point?) Molality doesn’t change w/temp of solution. Solution volume can expand/contract when you heat/cool it BUT – MASS of solvent won’t change when heated/cooled SO – the molaRity can change slightly when temp changes, but molaLity won’t. (explain using example)

Ex of molality calculations: a) Calculate the molality of a solution containing 10g NaCl in 500g H 2 O b) Calculate the molality of a solution containing 50g H 2 O in 800g ethanol

c) Calculate the molality when 75g of MgCl 2 is dissolved in 500g of solvent. d) 100g of sucrose (C 12 H 22 O 11, mol. wt. = g/mol) is dissolved in 1.50L of water. What is the molality?

e) Determine the molality of 3000g of solvent containing 37.3g of potassium chloride KCl f) How many grams of water must be used to dissolve 100 grams of sucrose C 12 H 22 O 11 to prepare a.2 molal solution?

CHALLENGE: How would you prepare 5000 grams of a.5 molal NaOH solution?

How would you prepare 3000g of a.3m NaCl solution? How would you prepare 1000g of a.75m LiCl solution?

Colligative Properties Colligative properties are properties that change when the concentration of the solution changes. Color, taste, density, BOILING POINT, FREEZING POINT

Colligative Properties When a solute dissolves in a solvent, the BOILING POINT of the resulting solution is ELEVATED from the original boiling point of the solvent by itself. The increase in the boiling temperature depends on the solute itself and how much of it is added (how concentrated the solution is). When a solute dissolves in a solvent, the FREEZING POINT of the resulting solution is DEPRESSED from the original boiling point of the solvent by itself. The decrease in the freezing temperature depends on the solute itself and how much of it is added (how concentrated the solution is).

Why does the type of solute used matter when trying to determine how much the boiling point and freezing point will change? IONIC COMPOUNDS DISSOCIATE (break apart) when they dissolve in water (due to molecule-ion attractions). The more particles that an ionic compounds splits into, the greater the effect on the boiling point and freezing point. This is called the van’t Hoff factor.

van’t Hoff factor NaCl splits into Na + and Cl -, or 2 particles. The van’t Hoff factor (i) is 2. MgCl 2 splits into Mg 2+ and two Cl -, or 3 particles. The van’t Hoff factor (i) is 3. NaNO 3 splits into Na + and NO 3 -, or 2 particles. The van’t Hoff factor (i) is 2. C 6 H 12 O 6 does NOT dissociate because it is MOLECULAR (covalent). The molecules stay together as intact units when the glucose dissolves. The van’t Hoff factor (i) is 1. *This is an oversimplified(!) way of determining the van’t Hoff factor, but it’ll do for our purposes. You’re welcome(?).

How to calculate the change in boiling point/freezing point of a solution: Boiling point elevation: ΔT b = iK b m ΔTb = change in the boiling point of the solvent i = van’t Hoff factor K b = the molal boiling point constant for the solvent (this will be given to you) m = molality of the solution

Freezing point depression: ΔT f = iK f m ΔT f = change in the freezing point of the solution i = the van’t Hoff factor K f = the molal freezing point constant for the solvent (this will be given to you) m = molality of the solution

Example calculations of boiling point elevation and freezing point depression: * the K b of water is.512 o C/molal * the K f of water is o C/molal

Calculate the boiling point elevation of a.3m solution of NaCl (aq). Calculate the freezing point depression of a.3m solution of NaCl (aq).

Calculate the boiling point elevation of a.5m solution of KNO 3(aq). Calculate the freezing point depression of a.5m solution of C 6 H 12 O 6(aq).

Calculate the boiling point elevation of a solution containing 25g CaCl 2 in 500g water.

Calculate the freezing point depression of a solution containing 50g C 6 H 12 O 6 in 500g water.