Solution Composition Concentration Mass Percent Molarity Mass Describes

Solution Composition & Concentration Mass Percent & Molarity!

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

Mass % example • A solution is prepared by dissolving 1. 0 g of sodium chloride in 48 g of water. Calculate the percent mass of the solution! – 1. 0 g of solute – Total mass of solution is 49 g (1. 0 g of solute + 48 g of solvent) 1. 0 g solute x 100% = 2. 0% Na. Cl 49 g solution

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 175 g of milk. 175 g milk X 0. 045 = 7. 9 g lactose

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. 0 M: “ 1. 0 molar”

Molarity example • A solution is prepared by dissolving 11. 5 g of solid Na. OH in enough water to make a 1. 50 L solution. (The solute and solvent together have a volume of 1. 50 L) Calculate the Molarity. – Mass of solute = 11. 5 g Na. OH convert to moles! – 11. 5 g Na. OH x 1 mol Na. OH = 0. 288 mol Na. OH 40. 0 g Na. OH – Volume of solution = 1. 50 L Molarity = 0. 288 mol Na. OH = 1. 92 M Na. OH 1. 50 L solution

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) Algebra g = (molar mass)(molarity)(volume) 58. 8 g potassium dichromate

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

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 m. L 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: