Unit 4 : Solutions

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: • Solution preparation from a solid solute • 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 • Do the math – find out what mass of solute you require • Mass out the solid into a beaker or onto a weighing paper • Dissolve the solid in approximately half of the total volume of the solution, in a beaker • Transfer the contents of the beaker to a volumetric flask

7. Rinse the beaker and stirring rod, and transfer that liquid to the volumetric flask • Carefully add water until the meniscus reaches the etched line • 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 c1v1 = c2v2 “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? c1v1 = c2v2 c1 = 2.0 mol/L v1 = ? L c2 = 0.15 mol/L v2 = 0.250 L v1 = 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? c1v1 = c2v2 c1 = 1.23 mol/L v1 = 400 mL = 0.400 L c2 = ? v2 = 0.70 L c2 = 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 • Do the math – find the volume of liquid required to make your solution • Measure that volume using a pipette (measure the meniscus properly) • Transfer the liquid to the volumetric flask • Carefully add water until the meniscus reaches the etched line • Stopper, and invert to mix

13. Homework • Worksheet