 # Molarity and Molality.

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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
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 = 3.0 mole 12.0L = 0.25 M

2. What is the molarity of a 400.0 mL solution containing 8.0 g NaOH?
Given: Mass of Solute – 8.0 g of NaOH Liters of Solution – mL Formula: Molarity = Moles of Solute Liters of Solution Answer: Moles = Mass of Solute Molar Mass of Solute = 8 g of NaOH ( g/mole) = 8 g NaOH 40 g/mole = 0.2 mole

Given: Molarity -. Liters of Solution – 400. 0 mL Moles of Solute – 0
Given: Molarity - ? Liters of Solution – mL Moles of Solute – 0.2 mole Formula: Molarity = Moles of Solute Liters of Solution Answer: M = Moles of Solute = 0.2 mole 0.4 L = 0.5 M = 400mL X 1 L 1000mL = 0.4L

3. How many grams of Ca(OH)2 are needed to prepare an 800
3. How many grams of Ca(OH)2 are needed to prepare an mL solution of M concentration? Given: Molarity – M Liters of Solution – mL Formula: Molarity = Moles of Solute Liters of Solution Answer: Moles = (Molarity) (Liters of Sol’n) = M (0.8 L) = moles = 800mL X 1 L 1000mL = 0.8L

Given: Mass of Solute –. Moles of Solute – 0
Given: Mass of Solute – ? Moles of Solute – mole Molar Mass – Ca(OH)2 = [40+([16x2]+[1x2])] = 74g/mole Formula: Moles = Mass of Solute Molar Mass of Solute Answer: Moles = Mass of Solute Mass of Solute = (Moles) (Molar Mass of Solute) = (0.012moles) (74g/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
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 = 0.46 mole 2.0kg = 0.23 m

2. What is the molality of a solution containing 75
2. What is the molality of a solution containing 75.5 g Pb(NO3)2 in g water? Given: Mass of Solute – 75.5 g Pb(NO3)2 Molar Mass = [207+(2)+(6)] = 331 g/mole Formula: Moles = Mass of Solute Molar Mass of Solute Answer: Moles = Mass of Solute = 75.5 g 331 g/mole = 0.23 moles

= 0.58 m Given: Molality – ? Moles of Solute – 0.23 moles
Kilogram of Solvent – g = 0.4kg Formula: m = Moles of Solute Kilogram of Solution Answer: m = Moles of Solute = 0.23 moles 0.4 kg = 0.58 m

What mass of solute is needed to prepare each of the following solutions?  a L of M K2SO4   b. 375 mL of M NaF   Calculate the molarity of each of the following solutions: a g KCl in mL solution  b g CaCl2 in L solution   Calculate the molality of each of the following solutions: a g of NaCl dissolved in g of water b mol KCl in 16.0 mol of H2O