December 3, 2009

Giving Directions

Making solutions ----> figure out mass to add

EXAMPLES:

A.Kristina is asked to make a 350mL solution of K2SO4 with a concentration of 0.65M. What STEPS will he follow?
V= 350mL = .350L

Molar Mass: K2SO4 = 174.3g/mol

0.65mol/L x (0.350L) = 0.2275

(o.2275mol) x 174.3g/mol = 39.65g

These are the STEPS we get when we refer to the work above:
1. Weigh 39.65 of K2SO4
2. Measure 350mL of water
3. Stir your solution


B. Give directions to make 3.00L of 5.0M NaOH.
Molar Mass: NaOH = 40g

5.0mol/L x (3.00L) = 15mol

(15mol) x 40g/mol = 600g

These are the STEPS we get when we refer to the work above:
1. Weigh 600g of NaOH
2. Measure 3.00L of water
3. Stir your solution

Dilution of Solution

When you add water the concentration decreases.
If volume is doubled, concentration is halved.



EXAMPLES:

A.Heidi adds 200.0mL of water to 60.0mL of 0.80M HCl. Find the final con'c of HCl.
C1 = 0.80M
V1 = 60.0mL
C2 = ?
V2 = 260mL

0.80M/L x (0.060L) = 0.048 mol

C1V1/V2 = C2 = (0.80)(60.0mL)/260mL = 0.18M


B. Alanna adds water to 200mL of 0.45M HF. He gets a final volume of 500mL. Find the final[HF].
C1 = 0.45M
C2 = ?
V1 = 200mL
V2 = 500mL

C1V1 = C2V2
(0.45)(0.200) = C2(0.500L)

(0.45)(0.200)/0.500 = C2

C2 = 0.10M

C. Sara dilutes 60.0mL of 0.40M HNO3 to 0.15M. What is the final volunme?

C1 =0.40M
V1 =60.0mL
C2 =0.15M
V2 =?

C1V1 = C2V2

(0.40M)(0.060L)/0.15M = V2
V2 = 0.160L = 160mL

-->How much water was added?
160mL - 60mL = 100mL