Solutions – Chapter 16 Mr.Yeung

Lesson 4 - Objectives Take up questions Must concentrate….in liquid form! And gas (Concentrations) Molarity

(Molarity) = moles of solute / litres of solution Example: putting 2 marbles in a can of Coke. The concentration would be: –2 marbles / a can of Coke –In mole terms… if you have 0.5moles and you want to find the molarity. It would be –0.5moles / 1L =.25M Molarity can sometimes be denoted as C for concentration or M for molarity Units for molarity = moles/litre

Questions Example 1. What is the molarity of a 5.00 liter solution that was made with 10.0 moles of KBr ? # of moles of solute Molarity = Liters of solution Given: # of moles of solute = 10.0 moles Liters of solution = 5.00 liters 10.0 moles of KBr Molarity = = 2.00 M 5.00 Liters of solution Answer = 2.00 M

Questions Example 2. A 250 ml solution is made with 0.50 moles of NaCl. What is the Molarity of the solution? We must change the ml to Liters as shown below: 250 ml 1 liter x = 0.25 liters 1000 ml # of moles of solute Molarity = Liters of solution Given: Number of moles of solute = 0.50 moles of NaCl Liters of solution = 0.25 L of solution 0.50 moles of NaCl Molarity = = 2.0 M solution 0.25 L Answer = 2.0 M solution of NaCl

Questions Example 1. What would be the volume of a 2.00 M (moles/L) solution made with 6.00 moles of LiF? Solution: # of moles of solute Liters of solution = Molarity Given: # of moles of solute = 6.00 moles Molarity = 2.00 M (moles/L) Liters of solution = 6.00 moles moles/L Answer = 3.00 L of solution

Questions Example 1. How many moles of CaCl2 would be used in the making L of a 5.0M solution? How many grams of CaCl2 were used? Solution: # of moles of solute = Molarity x Liters of solution. Given: Molarity = 5.0 M (moles/L) Volume = L # of moles of CaCl2 = 5.0 moles/L x moles Answer = 2.5 moles of CaCl2

Example 2. What is the volume of 3.0 M solution of NaCl made with 526g of solute? Solution: First find the molar mass of NaCl. Na = 23.0 g x 1 ion per formula unit = 23.0 g Cl = 35.5 g x 1 ion per formula unit = 35.5 g g Now find out how many moles of NaCl you have: mass of sample # of moles = Molar mass Given: mass of sample = 526 g Molar mass = 58.5 g 526 g # of moles of NaCl = g Answer: # of moles of NaCl = 8.99 moles Finally, go back to your molarity formula to solve the problem: # of moles of solute Liters of solution = Molarity Given: # of moles of solute = 8.99 moles Molarity of the solution = 3.0 M (moles/L) 8.99 moles # of Liters of solution = moles/L Final Answer = 3.0 L

Various representations of concentration For liquids- –Grams per litre (g/L) –%w/v = (mass of solute/100ml of solution) * 100 -%v/v = (volume of solute (ml)/100ml of solution) * 100 For solids %w/w = (mass of solute/100g of solution) * 100

Representations of concentrations For gas –Ppm = parts per million –Ex: 100 ppm sodium ions in water = 10 sodium ions in 1 million particles of water –Ppb = parts per billion –Ex: 10 ppb iron in water = 10 particles of iron in 1 billion particles of water