8.3 The Concentration of Solutions

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)

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)

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?

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%

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?

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

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 0.129 g of magnesium ion, the concentration of Mg2+ in the seawater is 0.129% (m/m)

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 23.47 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?

Solution: Percent (m/m) = mass of solute (in g) x 100% mass of solution (in g) = 4.58 g x 100% 23.47 g = 19.5% Therefore the mass/mass percent was 19.5% (m/m). 19.5 g of calcium chloride was dissolved in 100 g of solution

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.

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?

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.

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.

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?

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

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

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?

Solution: moles of NaCl = 0.90 g 58.44 g/mol = 1.54 x 10-2 mol Volume = 100 mL x 1.000 L 1000 mL = 0.100 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.

Using Molar Concentration to Find Mass At 20°C, a saturated solution of calcium sulfate, CaSO4, has a concentration of 0.0153 mol/L. A student takes 65 mL of this solution and evaporates it. What mass (in g) is left in the evaporating dish?

Solution: Volume (in L) =65 mL x 1.000 L 1000 mL = 0.065 L C = n/V n = 0.0153 mol/L x 0.065 L = 9.94 x 10-4 mol m (g) of CaSO4 = 9.94 x 10-4 mol x 136 g/mol = 0.135 g Therefore, 0.14 g of CaSO4 is left in the evaporating dish.

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