Measuring the Concentration of Solutions

There are several different ways to report the concentration of a solution, depending on factors: the states of matter of reactants and products, Whether or not you’re calculating the molar mass for a solute How accurate a measure of the concentration that you want/observe, and considerations of any further operations to be done with these values

Mass/Volume relationship : Measured as a percent: mass of solute (in grams) volume of solvent (in mL) x 100% Useful for solid solutes dissolved in liquid solvent – often an unsaturated solution. Simplifies calculations, don’t need to worry about moles

Mass/Volume Relationship: % mass/volume (often abbreviated “w/v” for “weight/volume”) is very common measurement in labs for reagents dissolved in several different solvents. e.g. Uses in my biology research lab: - Solid growth media for plants and bacteria (0.8 % agar) - Electrophoresis gels to separate DNA fragments (0.5 – 1.5 % agarose) - Sugar solutions (up to % glucose, fructose, sucrose etc.)

Mass/Volume Relationship: Prepare 100 mL of a 0.8 % (w/v) agar solution for the purpose of making growth media. 0.8 % = 0.8 g / 100 mL 100 mL agar solution x 0.8 g agar 100 mL agar solution = 0.8 g agar Pour 0.8 g agar to empty flask, add 100 mL H 2 O, warm on high heat, on hotplate, until agar is dissolved. Then pour plates.

Quick Demo Arrange yourselves in groups of 5 or 6 Measure out 2.0 grams of D-glucose into an empty flask. Obtain 20 mL cold tap water in a graduated cylinder. Make careful note of where the meniscus Is located, just before pouring Pour the water into the flask with the glucose Carefully mix until all glucose is dissolved. Pour glucose solution (1.0 gram / 10 mL, or 10 % w/v) back into graduated cylinder. Make note of location of meniscus.

Volume / Volume Relationship % volume/volume (often abbreviated “v/v”) Used when dissolving one liquid into another, for instance when diluting alcohols in water. e.g. 100 % (v/v) isopropyl alcohol is useful for precipitating DNA from a salt solution, while 70 % (v/v) ethyl alcohol in water is used to wash a precipitated DNA pellet Measured as a percent: Volume of solute liquid (in mL) x 100% Total Volume of liquid (in mL)

Mass/Mass Relationship Expressed as % mass (in grams)/mass (in grams) (often abbreviated “w/w” for “weight/weight”) Most often used for metal alloys. e.g. bronze is 80 % (w/w) Copper, 10 % (w/w) Zinc, and 10% (w/w) Tin.

Parts per million (ppm) and Parts per billion (ppb) Measured as a mass/mass relationship when a very small quantity of solute is present in a solvent. Not a measure of the relative number of particles. e.g. Our atmosphere contains about 350 – 400 ppm carbon dioxide Mass of carbon dioxide Mass of all atmospheric gases x 100%