Solution Concentrations

Ways of expressing concentration Fingers of whiskey Teaspoons of sugar Molarity Grams per L [dm3] % (W/V) [weight per volume] % (V/V) [volume per volume] % (W/W) [weight per weight] ppm (mg L-1) [Parts Per Million]

Molarity Moles are a chemists measure Moles per Litre i.e mol L-1 A 1 molar (1M) solution contains 1 mole of solute in 1 litre of solution.

Making a 1 molar [1M] solution Weigh out 1 mole of solute and dissolve it in some deionised water. Transfer the solution to a volumetric flask [with washings] Make up the solution to the one litre mark with deionised water. Long narrow neck makes it accurate Must mix it to make it homogeneous

We rarely make up a litre of solution [school is too mean to spend the money] 500 cm3 or 250 cm3 is much more usual Rarely make up 1 Molar solution might be 0.1 M or 0.02 M etc. Some examples

Problems What is the molarity of these solutions? 8 g NaOH in 200 cm3 of solution 5 g KMnO4 in 600 cm3 of solution 18 g K2Cr2O7 in 400 cm3 of solution 20 g HNO3 in 100 cm3 of solution 25 g CuSO4.5H2O in 50 cm3 of solution.

Grams per litre [dm3] Less scientific grams of solute per litre of solution Concentration = Molarity * Molar Mass

Example What is the concentration of 0.2 M NaCl? 0.2 mole = 0.2 of 58.5 = 11.7 g L-1 Conc. = Molarity * RMM

Problems What is the concentration of the following solutions in grams per L ? 0.1 M NaOH 0.5 M H2SO4 0.05 M KMnO4 10 M NH4OH 2.5 M HNO3

Percentage weight / volume % (W/V) Grams of solute per 100 cm3 of solution. E.g. what is the % (W/V) of 0.1 M NaOH 0.1 M NaOH = 0.1 of 40g per litre = 4 g per litre = 4 g per 1000 cm3 = 0.4 g per 100 cm3 = 0.4 % (W/V)

Percentage volume / volume % (V/V) Volume of solute per 100 cm3 of solution 10% (V/V) 10 cm3 Orange juice /100 cm3 solution Whiskey 40% (V/V) 40 cm3 Ethanol in 100 cm3 whiskey

How much alcohol?? Beer = 4% - Pint = 600 ml Wine = 12% - Glass = 140 ml Whisky = 40 % - Shot = 50 ml Volume of alcohol Beer = 4 * 6.0 = 24 ml Wine = 12 * 1.4 = 17 ml Whisky = 4 * 5.0 = 20 ml

Percentage weight per weight % (W/W) Grams of solute per 100 g of solution. When a powdered solid is dissolved in a powdered solid. Not common Self raising flour - Flour and baking soda Sensodyne 5%(W/W) KNO3

Parts per Million - ppm Measuring very dilute solutions e.g. pesticide residues in water or hardness mg per litre [milligrams per litre] a litre of water contains 1000g which is 1000000 milligrams So it tells us how many milligrams of solute there are in a million milligrams of solution

Problem A solution contains 0.035 g of calcium carbonate in 500 cm3. What is its hardness in p.p.m.? 0.035g is 35 mg 35 mg in 500 cm3 70 mg in 1000 cm3 = 70 ppm.