Measuring the Concentration of Solutions

1. Measuring the Concentration of Solutions

2. 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

3. 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

4. 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 20-30 % glucose, fructose, sucrose etc.)

5. 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 H2O, warm on high heat, on hotplate, until agar is dissolved. Then pour plates.

6. 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.

7. Volume / Volume Relationship Measured as a percent: Volume of solute liquid (in mL) x 100% Total Volume of liquid (in mL) % 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

8. 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.

9. 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%