1. Solution Composition & Concentration Mass Percent & Molarity!

2. Mass % Describes the composition of a solution. a.k.a. weight percent Mass % = mass of solute x 100% mass of solution = grams of solute x 100% grams of solute + grams of solvent

3. Mass % example A solution is prepared by dissolving 1.0g of sodium chloride in 48g of water. Calculate the percent mass of the solution! – 1.0 g of solute – Total mass of solution is 49g (1.0g of solute + 48g of solvent) 1.0g solute x 100% = 2.0% NaCl 49 g solution

4. Try this one… Cow’s milk typically contains 4.5% by mass of lactose (C 12 H 22 O 11 ). Calculate the mass of lactose present in 175g of milk. 7.9g lactose 175 g milk X 0.045 =

5. Molarity, M Describes the solution concentration in terms of volume – rather than composition in terms of mass (it’s much more convenient to measure the volume of a liquid!) M = Molarity = moles of solute = mol liters of solution L Ex/ a solution that contains 1.0 mol of solute per Liter of solution is 1.0M: “1.0 molar”

6. Molarity example A solution is prepared by dissolving 11.5g of solid NaOH in enough water to make a 1.50L solution. (The solute and solvent together have a volume of 1.50L) Calculate the Molarity. – Mass of solute = 11.5 g NaOH  convert to moles! – 11.5g NaOH x 1 mol NaOH = 0.288 mol NaOH 40.0g NaOH – Volume of solution = 1.50L Molarity = 0.288 mol NaOH = 1.92 M NaOH 1.50 L solution

7. Try this one… What mass of solid potassium dichromate (K 2 Cr 2 O 7 ) must be weighed out to make 1.00 L of a 0.200 M solution? (molar mass = 294.2) 58.8 g potassium dichromate Algebra g = (molar mass)(molarity)(volume)

8. Standard Solutions A solution with an accurately known concentration Prepared in a volumetric flask – Vol. flasks measure volume extremely accurately – Solute is weighed and completely added to vol. flask – Solvent is added to flask to bring volume up to exact volume line on neck of flask

9. How to make a solution of a given molarity. 1. Calculate the grams needed g = (mm)(M)(L) 2.Measure out that exact amount. 3.Rinse into mL volumetric flask. 4.Fill 2/3’ s full with distilled water. 5.Swirl to dissolve. 6.Fill to line with distilled water. 7.Cap and flip 20 times. 8.Label: M(what it is), date:, who made it: