Two methods for Preparation of a desired volume of a Molar Solution  1) Preparation from a solid solute.  2) Preparation by Dilution of a Concentrated Stock Solution.

Molarity Definition  Molarity (M) = moles of solute Liter of solution 58.5 g 1 M NaCl solution: 1 Mole (58.5 g) of NaCl dissolved in a total aqueous solution volume of 1 Liter.

Solution = Solute + Solvent Important Note: Unless otherwise specified, assume that the solvent for all solutions discussed in this course is WATER.

What volume of water would be required to prepare 1 liter of a 1 M NaCl solution?  A) Less than 1 liter  B) Exactly 1 liter  C) More than 1 liter ANS: A – Less than 1 liter. Solution = Solute (NaCl) + Solvent (H 2 O). The solute takes up space when it dissolves, thus less than 1 liter of water is required.

Preparation of a desired volume of solution from a solid solute.  Calculate moles of solute needed.  Calculate grams of solute needed.  Weigh solute with scale, and transfer to volumetric flask calibrated to prepare desired volume.  Rinse container with water to make sure that solute is transferred  Dissolve solute in small volume of water.  Fill up to mark with water.

Preparing Molar Solution From Solid Give detailed directions for the preparation of 250. mL of a M solution of CuCl 2 from solid CuCl 2. Required Materials: CuCl 2 Scale, small beaker, spatula 250. mL volumetric flask Water (wash bottle)

Preparing Molar Solution From Solid Give detailed directions for the preparation of 250. mL of a M solution of CuCl 2 from solid CuCl 2. Step 1: Calculate moles: How could you determine the # of moles from the given information?

Calculating Moles – mathematical vs. conceptual approach Rearrange Molarity Equation Molarity = Moles Solute Liters solution Solve for moles by multiplying by liters: M = x moles L x = M x L = moles Proportion Method Scale up or down moles in 1 liter by using a proportion : x moles = moles.250 L 1.00 L x = M x L = moles Moles = Molarity x Liters 1 liter 0.25 L How many in 0.25 L

Preparing Molar Solution From Solid Give detailed directions for the preparation of 250. mL of a M solution of CuCl 2 from solid CuCl 2. Step 1: Calculate moles: moles = Molarity x volume in liters. Converting mL to L: 250.mL (1.00 L) = L ; L (0.100 moles CuCl 2 ) = moles CuCl 2 Molarity = moles solute Liters of solution (1000. mL) (1.00 L)

Preparing Molar Solution From Solid Give detailed directions for the preparation of 250. mL of a M solution of CuCl 2 from solid CuCl 2. Step 2: Convert moles to grams: moles CuCl 2 → ? g Cu: 63.5 Cl 2 : 2 x 35.5 = g/mol moles CuCl 2 ( g CuCl 2 ) = 3.59 g CuCl 2 (1 mole CuCl 2 ) Molarity = moles solute Liters of solution

Preparation of Solutions Weigh 3.59 g CuCl 2 ; transfer to 250 mL volumetric flask Dissolve CuCl 2 in a small volume of water Add water to line

Link to Preparation of Molar Solution Video

Dilution of Solutions

Preparation of a desired volume of a Molar Solution from Dilution of a Concentrated Stock Solution  Calculate volume of stock solution needed using Dilution Equation: M C V C = M D V D  Transfer (using pipette or graduated cylinder) required volume of stock solution to volumetric size calibrated to prepare desired volume.  Dilute with water up to calibration mark.

Dilution of solutions  Since moles are constant before and after dilution, we can use the following formula for calculations. M C V C = M D V D M C = Molarity of Concentrated solution V C = Volume of concentrated stock M D = Molarity of Dilute solution V D = Volume of Dilute of solution Note: M C > M D

The Dilution formula E.g. if we have 1 L of 3 M HCl, what is M if we dilute acid to 6 L? M 1 = 3 mol/L, V 1 = 1 L, V 2 = 6 L M 1 V 1 = M 2 V 2, M 1 V 1 /V 2 = M 2 M 2 = (3 mol/L x 1 L) / (6 L) = 0.5 M Why does the formula work? Because we are equating mol to mol: M 1 V 1 = 3 mol V 1 = 1 L M 1 = 3 M V 2 = 6 L M 2 = 0.5 M M 2 V 2 = 3 mol

Dilution of Solutions  Describe how you would prepare 250. mL of a M CuCl 2 solution, starting with a M CuCl 2 stock solution. M c = 0.500M V C = ? M D = 0.100M V D = 250. mL M C V C = M D V D M x V C = M x 250. mL V C = M x 250. mL (0.500 M) V C = 50.0 mL of M CuCl 2 stock solution Dilute 50.0 mL of M CuCl 2 stock solution with enough water to prepare 250. mL of solution.

Diluting a Solution (a) Measure volume of concentrated stock (b) Transfer into volumetric Flask (c) Fill to line with water.