2 Molarity MOLARITY Let’s start our discussion of solution concentration Measurement withMOLARITYMolarity: the most common solution concentrationmeasurement is defined as follows:M = moles of soluteLITERS of solutionWhich means Molarity = # of moles per liter of solutionCapital M
3 Molarity Let’s Consider the following Problem: What is the molarity of a solution in which 50g ofCuSO4 is dissolved in water to make a 2 liter solution.Step 1. What’s the formula for MOLARITY?M = moles of soluteliters of solution
4 Molarity Step 2. We need to determine how many moles we have in 50g of CuSO4 .How do we do that? Remember the mole Hole?1. Determine the molar mass of CuSO4(159.5g/mol)2. Calculate the # of moles of CuSO4Since we’re going into the mole hole we needto DIVIDE # of grams by molar mass(50/159.5) = 0.31 moles
5 Molarity Step 3. Calculate the Molarity of the solution using the formula for MOLARITYM = moles of Soluteliters of solutionHow many moles do we have?How many liters do we have?So M = moles2 liters0.31 mol2 LM = mol/liter
6 Making a 1M NaCl Solution Weigh out and add 1 mole of NaCl to a volumetric FlaskAdd a small amount of solvent to dissolve the salt3. Once the soluteis dissolved addadditional solventup to the 1 litermark
7 Problem Calculate how many moles of ammonia there are What is the molarity of a solution made by dissolving 2.1liters of NH3 (g) in sufficient water to make 2 liters ofsolution?What do we need to begin calculating the Molarity of the solution?Calculate how many moles of ammonia there are# of moles = volume = 2.1 liters = molmolar vol l/mol
8 Problem Step 2. Calculate the molarity of the solution We now know the number of moles of NH3 we have (0.094) so …Step 2. Calculate the molarity of the solutionusing the molarity equationM = # moles = mol = M# liters of sol’n liters
9 Molarity by dilution Note that it is only possible to go from a more Now what if we had a solution already prepared butWanted to decrease the molarity to some lower preciseValue.How could we use an already prepared higher molaritysolution to accomplish this?Note that it is only possible to go from a moreconc. (higher molarity) solution to a less conc.(lower molarity) solution.concdil
10 Molarity by dilutionTo make something more dilute we’re going to need toadd more solvent to the solution.Recall that Molarity (M) = # moles of solute# liters of solutionSo we need to dilute the solution by adding solventBut how much more solvent do we need to add?
11 Molarity by dilution When we add more solvent to a solution do we change the number of moles of solute in thesolution?NO! But what does change?We change the # of molesper unit volume of Solvent becausethe same # of moles are present butnow there’s more liquid
12 Molarity by Dilution Let’s see how this works … So… #moles = M1 x V1 Let’s call the Molarity of the original solution M1 and theNew Molarity M2The number of liters of the original solution and the newSolution will then be designated by V1 and V2So the molarity of the old solution is…M1 = #moles of solute = #moles#liters of solution V1So… #moles = M1 x V1
13 Molarity by Dilution The Molarity of the new solution will then be … M2 = #moles of solute#liters of solution# moles = M2 x V2Remember that the # of moles does not change whenWe dilute solutions.
14 Molarity by dilution So… M1 x V1 = M2 x V2 If we know three values we can calculate the 4th.Let’s see how we use this relationship
15 Dilution Problem How much of a 3M solution of NiCL2 is needed to Prepare 100 ml of a 1.75M solution of NiCl2?OK how do we solve this? What’s the equationwe’re going to use?Right! M1V1 = M2V2M1 = 3M; M2 = 1.75M; V2 = 0.1L (100ml)So … 3 x V1 = 1.75 x 0.1V1 = 0.058L or 58ml
16 Molarity by dilution Add 4.375L H20 to a 0.625L of a 12M sol’n How do we make 5L ofA 1.5M sol’n of KCLFrom 12.0M stock sol’n?M1V1 = M2V212 x V1 = 1.5 x 512V1= 7.5V1 = 7.5/12 = 0.625LAdd 4.375L H20 to a0.625L of a 12M sol’n