1. 8.3 The Concentration of Solutions

2. The Concentration of Solutions
Concentration: The amount of solute per quantity of solvent.
Chemicals lethal at high concentrations can still be used in everyday items.
(depending on the concentration)

3. Concentration as a Mass/Volume Percent
Mass/volume percent: mass of solute dissolved in a volume of solution, expressed as a percent
The number of grams of solute per 100 mL of solution is numerically equal to the mass/volume percent.
E.g. 0.9 g NaCl in 100 mL of solution is 0.9% (m/v)

4. Solving for Mass/Volume Percent
A pharmacist adds 2.00 mL of distilled water to 4.00 g of a powdered drug. The final volume of the solution is 3.00 mL. What is the percent (m/v) of the solution?

5. Solution:
Percent (m/v) = mass of solute (in g) x 100% volume of solution (in mL) = 4.00 g x 100% 3.00 mL = 133%

6. Finding m for an (m/v) concentration
The recommended concentration of trisodium phosphate, Na3PO4 is 1.7% (m/v). What mass of TSP is needed to make 2.0 L of solution?

7. Solution:
Percent (m/v) = mass of solute (in g) x 100% volume of solution (in mL) Mass of solute = percent (m/v) x volume of solution 100 % = 1.7% x 2000 mL 100% = 34 g

8. Concentration as a Mass/Mass Percent
Mass/mass percent: mass of a solute divided by the mass of solution, expressed as a %.
The number of grams of solute per 100 g of solution is numberically equal to the m/m %
E.g. If 100 g of seawater contains g of magnesium ion, the concentration of Mg2+ in the seawater is 0.129% (m/m)

9. Solving for a m/m %
To determine how much calcium chloride had been used on a nearby road, a student took a sample of slush to analyze. The sample had a mass of g. When the solution was evaporated, the residue had a mass of 4.58 g. (Assume that no other solutes were present.) What was the mass/mass percent of calcium chloride in the slush? How many grams of calcium chloride were present in 100 g of solution?

10. Solution:
Percent (m/m) = mass of solute (in g) x 100% mass of solution (in g) = 4.58 g x 100% g = 19.5% Therefore the mass/mass percent was 19.5% (m/m) g of calcium chloride was dissolved in 100 g of solution

11. Concentration as a v/v %
Volume/volume percent: volume of solute divided by the volume of solution, expressed as a %.
Also referred to as the volume by % concentration, volume percent, percent (v/v), or the % by volume.

12. Solving for v/v %
Rubbing alcohol is sold as a 70% (v/v) solution of isopropyl alcohol in water. What volume of isopropyl alcohol is used to make 500 mL of rubbing alcohol?

13. Solution:
Percent (v/v) = Volume of solute x 100% Volume of solution Volume of solute = percent (v/v) x volume of solution 100 % = 70% x 500 mL 100% = 350 mL Therefore, 350 mL of isopropyl alcohol is used to make 500 mL of 70% (v/v) rubbing alcohol.

14. Concentration in ppm & ppb
Parts per million (ppm) and parts per billion (ppb): Mass/mass relationships that describe the amount of solute that is present in a solution.

15. ppb in Peanut Butter
A fungus that grows on peanuts produces a deadly toxin. Any shipment of peanuts that contains more than 25 ppb of this dangerous fungus is rejected. A company receives 20 t of peanuts to make peanut butter. What is the maximum mass (in g) of fungus that is allowed?

16. Example Problem 5 Parts per Billion in Peanut Butter
20 t x 1000 kg/t x 1000 g/kg = 20 x 106 g ppb = Mass of fungus x 109 Mass of peanuts Mass of fungus = ppb x Mass of peanuts 109 = 25 ppb x (20 x 106 g) = 0.5 g Therefore, the maximum mass of fungus that is allowed is 0.5 g

17. Molar Concentration
Molar concentration: The number of moles of solute that can dissolve in 1L of solution. Also known as molarity
C = n / V

18. Calculating Molar Concentration
A saline solution contains 0.90 g of sodium chloride, NaCl, dissolved in 100 mL of solution. What is the molar concentration of the solution?

19. Solution:
moles of NaCl = 0.90 g g/mol = 1.54 x 10-2 mol Volume = 100 mL x L 1000 mL = L C = n / V C = 1.54 x 10-2 mol/0.100L C = 1.54 x 10-1 mol/L Therefore the molar concentration of the saline solution is 0.15 mol/L.

20. Using Molar Concentration to Find Mass
At 20°C, a saturated solution of calcium sulfate, CaSO4, has a concentration of mol/L. A student takes 65 mL of this solution and evaporates it. What mass (in g) is left in the evaporating dish?

21. Solution:
Volume (in L) =65 mL x L 1000 mL = L C = n/V n = mol/L x L = 9.94 x 10-4 mol m (g) of CaSO4 = 9.94 x 10-4 mol x 136 g/mol = g Therefore, 0.14 g of CaSO4 is left in the evaporating dish.

22. Homework:
P 318 #1-4 McGraw Hill
P 441 # 1-10 Nelson