Unit 4 : Solutions8.4 – Dilution and Solution Preparation

Context

•Remember: c=n/v

•Why would this be an important formula to know for chemistry?

•Lab Work: Preparation of Solutions!!

Solution Preparation

•Solutions need to be known for multiple reasons

•Concentrations need to be known

•Two methods:1. Solution preparation from a solid solute2. Solution preparation from a liquid solute

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

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

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

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

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

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/Lv1 = ? Lc2 = 0.15 mol/Lv2 = 0.250 L

v1 = 0.0188 L = 18.8 mL

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/Lv1 = 400 mL = 0.400 Lc2 = ?v2 = 0.70 L

c2 = 0.703 mol/L

Note

•Check for the “reasonableness” of answers

• If you are diluting something, the final concentration will be LESS

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

Homework

•Worksheet