Molarity and Molality Molarity M number of moles

Molarity and Molality

Molarity (M) number of moles of solute (n) per liter (L) of solution Molarity (M) = Moles of Solute Liters of Solution Note: Moles = Mass of Solute Molar Mass of Solute

1. What is the molarity of the solution if 3. 0 mole of solute is dissolved in 12. 0 L solution? Given: Moles of Solute – 3. 0 mole Liters of Solution – 12. 0 L Formula: Molarity = Moles of Solute Liters of Solution Answer: M = Moles of Solute Liters of Solution = 3. 0 mole 12. 0 L = 0. 25 M

2. What is the molarity of a 400. 0 m. L solution containing 8. 0 g Na. OH? Given: Mass of Solute – 8. 0 g of Na. OH Liters of Solution – 400. 0 m. L Formula: Molarity = Moles of Solute Liters of Solution Answer: Moles = Mass of Solute Molar Mass of Solute = 8 g of Na. OH (23+16+1 g/mole) = 8 g Na. OH 40 g/mole = 0. 2 mole

Given: Molarity - ? Liters of Solution – 400. 0 m. L Moles of Solute – 0. 2 mole Formula: Molarity = Moles of Solute Liters of Solution Answer: M = Moles of Solute Liters of Solution = 400 m. L X 1 L = 0. 2 mole 1000 m. L 0. 4 L = 0. 4 L = 0. 5 M

3. How many grams of Ca(OH)2 are needed to prepare an 800. 0 m. L solution of 0. 015 M concentration? Given: Molarity – 0. 015 M Liters of Solution – 800. 0 m. L Formula: Molarity = Moles of Solute Liters of Solution Answer: Moles = (Molarity) (Liters of Sol’n) = 0. 015 M (0. 8 L) = 800 m. L X 1 L = 0. 012 moles 1000 m. L = 0. 8 L

Given: Mass of Solute – ? Moles of Solute – 0. 012 mole Molar Mass – Ca(OH)2 = [40+([16 x 2]+[1 x 2])] = 74 g/mole Formula: Moles = Mass of Solute Molar Mass of Solute Answer: Moles = Mass of Solute Molar Mass of Solute = (Moles) (Molar Mass of Solute) = (0. 012 moles) (74 g/mole) = 0. 89 g

Molality (m) as the number of moles of solute (n) per kilogram (kg) of solution Molality (m) = Moles of Solute Kilograms of Solvent Note: Moles = Mass of Solute Molar Mass of Solute

1. What is the molality of a solution containing 0. 46 mole of solute in 2. 0 kg water? Given: Moles of Solute – 0. 46 mole Kilogram of Solvent – 2. 0 kg Formula: Molality = Moles of Solute Kilogram of Solvent Answer: m = Moles of Solute Kilogram of Solvent = 0. 46 mole 2. 0 kg = 0. 23 m

2. What is the molality of a solution containing 75. 5 g Pb(NO 3)2 in 400. 0 g water? Given: Mass of Solute – 75. 5 g Pb(NO 3)2 Molar Mass = [207+([14]2)+([16]6)] = 331 g/mole Formula: Moles = Mass of Solute Molar Mass of Solute Answer: Moles = Mass of Solute Molar Mass of Solute = 75. 5 g 331 g/mole = 0. 23 moles

Given: Molality – ? Moles of Solute – 0. 23 moles Kilogram of Solvent – 400. 0 g = 0. 4 kg Formula: m = Moles of Solute Kilogram of Solution Answer: m = Moles of Solute Kilogram of Solution = 0. 23 moles 0. 4 kg = 0. 58 m

1. What mass of solute is needed to prepare each of the following solutions? a. 1. 00 L of 0. 125 M K 2 SO 4 b. 375 m. L of 0. 015 M Na. F 2. Calculate the molarity of each of the following solutions: a. 12. 4 g KCl in 289. 2 m. L solution b. 16. 4 g Ca. Cl 2 in 0. 614 L solution 3. Calculate the molality of each of the following solutions: a. 2. 89 g of Na. Cl dissolved in 0. 159 g of water b. 1. 80 mol KCl in 16. 0 mol of H 2 O