1. Unit 4 : Solutions 8.4 – Dilution and Solution Preparation

2. Context Remember: c=n/v Why would this be an important formula to know for chemistry? Lab Work: Preparation of Solutions!!

3. Solution Preparation Solutions need to be known for multiple reasons Concentrations need to be known Two methods: 1.Solution preparation from a solid solute 2.Solution preparation from a liquid solute

4. Solution Terms “Standard Solution”: a solution which you have made, and for which you know the accurate concentration “Stock Solution”: a concentrated solution that you make or buy, and then will use to make more dilute solutions

5. Equipment Solutions are made in volumetric flasks Volumetric flasks and pipettes are very, very accurate pieces of glassware ▫Look on the vessel for +/- accuracy, generally +/- 0.1mL Graduated cylinders and pipettes have intermediate accuracy Beaker are NOT actually for measuring volume

6. Solutions from Solids 1.Do the math – find out what mass of solute you require 2.Mass out the solid into a beaker or onto a weighing paper 3.Dissolve the solid in approximately half of the total volume of the solution, in a beaker 4.Transfer the contents of the beaker to a volumetric flask

7. 5.Rinse the beaker and stirring rod, and transfer that liquid to the volumetric flask 6.Carefully add water until the meniscus reaches the etched line 7.Stopper, and invert to mix

8. Solutions from liquid solute You want to dilute the stock solution you have, by adding more solvent … you are diluting the solution, which lowers the concentration c 1 v 1 = c 2 v 2 “1” means the initial conditions “2” means the final conditions ** New Total Volume

9. Example How much 2.0 M NaCl solution would you need to make 250 mL of 0.15 M NaCl solution? c 1 v 1 = c 2 v 2 c 1 = 2.0 mol/L v 1 = ? L c 2 = 0.15 mol/L v 2 = 0.250 L v 1 = 0.0188 L = 18.8 mL

10. Example 2 What is the final concentration of NaCl if 300mL of water is added to 400 mL of a 1.23 M solution of NaCl? c 1 v 1 = c 2 v 2 c 1 = 1.23 mol/L v 1 = 400 mL = 0.400 L c 2 = ? v 2 = 0.70 L c 2 = 0.703 mol/L

11. Note Check for the “reasonableness” of answers If you are diluting something, the final concentration will be LESS

12. Solutions for Liquid Stock Solutions 1.Do the math – find the volume of liquid required to make your solution 2.Measure that volume using a pipette (measure the meniscus properly) 3.Transfer the liquid to the volumetric flask 4.Carefully add water until the meniscus reaches the etched line 5.Stopper, and invert to mix

13. Homework Worksheet