After going over questions #25-28 from our Unit 2 Homework Questions, we took notes on Giving Directions and Dilution of Solutions.
GIVING DIRECTIONS
Outline for experimental procedures
---------> simply find the mass you need.
It isn't hard. You just convert from con'c---->moles---->mass
Remember that C=n/v (Concentration = number of moles over volume)
Then you give the directions like so, filling in the missing information:
a) Measure (Volume in L or mL) of Water
b)Weigh (mass in g) of (element or compound)
c) Add (element or compound) to water and stir until dissolved.
Example: Lizzy is asked to make a 3.00 M solution of NH4Cl. If she needs 1.00 L, what procedure should she use?
3.00 mol L x 1.00 L = 3.00 mol
3.00 mol mol x 53..5 g/1 mol = 161 g
a) Measure 1.00 L
b) Weigh 161 g of NH4Cl
c) Add NH4Cl to water and stir until dissolved
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DILUTION OF SOLUTIONS
When you add water, the con'c decreases. If the volume is doubled, the con'c is halved. If the volume is halved, the concentration is doubled.
6.0 L = 2.0 mol/L (Start out with)
12.0 L = 1.0 mol/L (Multiply the volume by 2, divide the volume by 2)
48.0 L = 0.25 mol/L (Multiply the volume by 4, divide the volume by 4)
The number of moles is doesn't change though (In this case 12.0 mol)
Remember that C1V1 = C2V2 (Where C1 is equal to the initial concentration, C2 is equal to the final concentration, V1 is equal to the initial volume and V2 is equal to the final volume)
Ex: If 20.0 mL of 0.75 M HBr is diluted to a total volume of 90.0 mL, what is the molar concentration of the HBr in the resulting solution?
V1 = 20.0 mL
C1 = 0.75 M
V2 = 90.0 mL
C2 = ?
(0.75) (20) = C2 (90)
C2 = 0.17 M
Here's a video on calculating Dilution of Solutions for some more help:
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