SCH 3U Concentration (pg. 281-291)

Benzoic acid, is commonly, used as a food preservative and as an ingredient in cosmetics, germicides, and medications for treating fungus infections. It occurs naturally in cranberries, and in most other types of berries. However, benzoic acid is moderately toxic. If too large a quantity is consumed, it causes abdominal pain, nausea and vomiting. When benzoic acid is added to food, the mass is typically no more than 0.1 percent of the mass of the food. The WHO recommends that people limit their daily consumption of benzoic acid to 5 mg /kg of body mass. Benzoic acid is not the only chemical that can be dangerous if the concentration is too great. The proportions of many chemicals in food, medicines and the body often determine whether the chemicals are harmful or beneficial.

Concentration:

There are several commonly used measurements to describe concentration. All of these measures express concentration in terms of unit quantities of solution.

Expressing Concentration

Qualitatively, a solution can be described as concentrated or dilute.

A concentrated solution contains many particles of solute per unit quantity of solution, whereas a dilute solution contains relatively few solute particles.

Comparison: concentration vs solubility

Concentration = solubility =

* solution includes solute and solvent * solvent refers only to the water or solvent (not including solute)

Ways to measure / report concentration:

Ratios of solute to solution are commonly expressed as percents or as molar concentrations.

There are many common methods for expressing concentrations

Type of concentration / Definition / Units / Common Application
mass/volume percent (%m/v) / g/100mL / Intravenous solutions such as a saline drip
volume percent (%v/v) / Solutions prepared by mixing liquids
mass percent (%m/m) / Concentration of metals in alloys
very low (number) / ppm, ppb (x109) ppt / Safety limits for contaminants, such as mercury or lead in food or water
molar / mol/L / Solutions used as reactants

Examples

1. Water is added to 21.0 mL of alcohol until the total volume of the mixture is 100.0mL. Find the volume percent of alcohol in the mixture.

2. An intravenous solution for a patient was prepared by dissolving 17.5 g of glucose in distilled water to make 350 mL of solution. Find the mass-volume percent concentration of the solution.

3. Find the mass of pure silver in a sterling silver ring that has a mass of 6.45 g. The concentration of silver in sterling silver alloys is 92.5% (m/m).

4. Health Canada’s guideline for the maxiumum mercury content in commercial fish is 0.5 parts per million (ppm). When a 1.6 kg salmon was tested, it was found to contain 0.6 mg of mercury. Would this salmon be safe to eat?

5. A solution contains 5.85 g of sodium chloride dissolved in 500 mL of water. What is the concentration of sodium chloride in mol/L ?

6. A student dissolved 0.212 mol of iron (III) chloride, to make a 175 mL solution. Find the molar concentration of the solution and the concentrations of the ions in solution.

Textbook practice: pg. 284 #2-6,8 pg. 287 #11-17, pg. 290 #19-22
Molerific practice: The Molarity of Solutions

1. What mass of the following chemicals is needed to make the solutions indicated?

a. 1.0 liter of a 1.0 M mercury (II) chloride (HgCl2) solution

b. 2.0 liters of a 1.5 M sodium nitrate (NaNO3) solution

c. 5.0 liters of a 0.1 M Ca(OH)2 solution

d. 100 mL of a 0.5 M (NH4)3PO4 solution

2. Calculate the molarity of the following solutions.

a. 12 g of lithium hydroxide (LiOH) in 1.0 L of solution

b. 198 g of barium bromide (BaBr2) in 2.0 L of solution

c. 54 g of calcium sulfide (CaS) in 3.0 L of solution

3. Calculate the volume of each solution, in liters.

a. a 1.0 M solution containing 85 g of silver nitrate (AgNO3)

b. a 0.5 M solution containing 250 g of manganese (II) chloride (MnCl2)

c. a 0.4 M solution containing 290 g of aluminum nitrate (Al(NO3)3)

4. How many grams of potassium chloride (KCl) are required to make 2.0 L of a 3.0 M solution?

5. How many grams of magnesium chloride (MgCl2) are needed to make 6.0 L of a 3.0 M solution?

6. What mass of barium chloride (BaCl2) is needed to make 0.5 L of a 4.0 M solution?

7. What mass of iron (II) sulfate (FeSO4) is needed to make 200 mL of a 0.25 M solution?

7. What mass of iron (II) sulfate (FeSO4) is needed to make 200 mL of a 0.25 M solution?

8. What is the molarity of a solution in which 1.6 g of sodium hydroxide (NaOH) are dissolved in 125 mL of solution?

9. What is the molarity of a solution in which 5.0 g of sodium carbonate (Na2CO3) are dissolved in 200 mL of solution?

10. How many grams of silver nitrate (AgNO3) are needed to make 2.0 L of a 0.10 M solution?

11. 2.0 L of a solution contain 25 g of potassium permanganate (KMnO4). What is the molarity of the solution?

12. How many grams of glycerine (C3H8O3) are needed to make 100 mL of a 2.5 M solution?

13. What is the molarity of a solution containing 150 g of zinc sulfate (ZnSO4) per liter?

14. A test tube contains 10 mL of a 3.0 M CaCO3 solution. Calculate the number of grams of CaCO3 in the tube.

Answers:

1a. 272 g HgCl2 3a. 0.50 L AgNO3 (aq) 8. 0.32 M NaOH (aq)

1b. 255 g NaNO3 3b. 4.0 L MnCl2 (aq) 9. 0.24 M Na2CO3 (aq)

1c. 37 g Ca(OH)2 3c. 3.4 L Al(NO3)3 (aq) 10. 34 g AgNO3

1d. 7.5 g (NH4)3PO4 4. 448 g KCl 11. 0.08 M KMnO4 (aq)

2a. 0.50 M LiOH (aq) 5. 1715 g MgCl2 12. 23 g C3H8O3

2b. 0.33 M BaBr2 (aq) 6. 417 g BaCl2 13. 0.93 M ZnSO4 (aq)

2c. 0.25 M CaS (aq) 7. 7.6 g FeSO4 14. 3.0 g CaCO3