Chemistry 30 – Unit 2 – Solubility – Ch. 16 in Text

More than just Molarity…  Remember, the word “concentration” refers to how much solute is dissolved in the solution. Molarity is just ONE WAY of measuring concentration, i.e. in number of moles/litre. There are other ways of measuring concentration: 1) Mass per Volume 2) Percent ○ A) Mass/volume ○ B) Volume/volume 3) Parts per million (ppm) 4) Parts per billion (ppb)

1. Mass per Volume Concentration  We can measure concentration as mass/volume (spoken as “mass per volume”).  The most common units for this are grams/litre (g/L, spoken as “grams per litre.”)  The formula for calculating a mass/volume concentration is C = m/v.  And remember, just like M 1 V 1 = M 2 V 2, C 1 V 1 = C 2 V 2.

1. Mass per Volume Concentration  Example #1 – g of Na 2 CO 3 is dissolved in 250 mL of water. What is the concentration in g/L? What is the molar concentration? Step One: C = m/vC = g/0.250L C = 22.7 g/L Step Two: ○ g x (1 mole/106 g) = mol ○ mol/0.250 L = M

2. Percent Concentration  Remember, “percent” literally means per 100 units No different than calculating up a percentage on an exam  Units must be in Grams (g) if you are dealing with mass Milliliters (mL) if you are dealing with volume  Formula: % = SOLUTE in g or mL (100) SOLUTION in g or mL

NOTE  “SOLUTION” and “SOLVENT” mean the same thing… they can be used interchangeably!

2. Percent Concentration  A) Mass/volume Percent (mass/volume) is the number of grams of solute per mL of solution. This measurement is often used when we are dissolving a solid solute (conveniently weighed in grams!) into a liquid solvent, measured in mL. Example #2A: A solution containing 7 g of sodium chloride in 100 mL solution is 7 percent (mass/volume), or in short form, 7% (m/v).

2. Percent Concentration  B) Volume/volume Percent (volume/volume) is the number of mL of solute per mL of solution. This is convenient when we are making a solution when both the solute and the solvent are liquids. Example #2B: If 20 mL of rubbing alcohol is diluted with water to a total volume of 100 mL, the final solution is 20% alcohol by volume (20 mL alcohol/100 ml total volume x 100 % = 20% v/v)

2. Percent Concentration  Pointers: Percent compositions CAN be misleading! When a label says a product contains 5% glucose, what does it mean? ○ It PROBABLY means percent (m/v), but we can’t be sure. So when you do a percent composition calculation, make sure you always indicate if you are using a (m/v) or a (v/v) calculation.

3. Parts per Million Concentration  Used for an extremely dilute solution  Parts per million, (or ppm) means, for every one million “parts” of the solution, how many of them are solute?  The formula for calculating ppm is much like the formula for calculating percent.  We can measure ppm in (m/v) or (v/v), or even (m/m)

3. Parts per Million Concentration  Formulas for calculating ppm: ppm (m/v) = [mass of solute (g) / volume of solution (mL)] x (1 x10 6 ) ppm (v/v) = [volume of solute (mL) / volume of solution (mL)] x (1 x10 6 ) ppm (m/m) = [mass of solute (g) / mass of solution (g)] x (1 x10 6 )

3. Parts per Million Concentration  Example #3 – What is the concentration in g/L and M of a sample of well water that has 20. ppm iron? Step One: Realize that ppm in this case means that for every mL of well water, there are 20 g Fe. convert ppm Fe into grams Fe. 20 ppm = 20. g Fe/ mL H 2 O = g/L Step Two: Convert g/L Fe into M: g/L Fe x (1 mole Fe/ g Fe) = M

4. Parts per Billion Concentration  Same idea as parts per million, except this time it’s parts per billion  Used for an super extremely dilute solution  Parts per billion, (or ppb) means, for every one billion “parts” of the solution, how many of them are solute?  The formula for calculating ppb is much like the formula for calculating percent.  We can measure ppb in (m/v) or (v/v), or even (m/m)

4. Parts per Billion Concentration  Formulas for calculating ppb: ppb (m/v) = [mass of solute (g) / volume of solution (mL)] x (1 x10 9 ) ppb (v/v) = [ volume of solute (mL) / volume of solution (mL)] x (1 x10 9 ) ppb (m/m) = [mass of solute (g) / mass of solution (g)] x (1 x10 9 )

NOTE  “SOLUTION” and “SOLVENT” mean the same thing… they can be used interchangably!

4. Parts per Billion Concentration  Example #4 - What is the concentration of Hg +2 in ppb, ppm, and % of a water sample that assays at 6.08 x M HgCl 2 ? Step One: Write and balance the dissociation formula: HgCl 2 (s)  Hg +2 (aq) + 2Cl - (aq) ○ The molar ratio between HgCl 2 (s) and Hg +2 (aq) tell us that for every 6.08 x M HgCl 2, there will be 6.08 x M Hg +2 ions. Step Two: Convert from M to g/L: 6.08 x mol/L x g/mol = 1.22 x g/L Step Three: Convert from g/L to ppm: 1.22 x g/1000 mL x (1 x 10 6 )= 1.22 ppm Step Four: Convert from g/L to ppb: 1.22 x g/1000 mL (1x 10 9 )= 1220 ppb Step Five: To determine % (m/v): 1.22 x g/1000 mL x 100 % = % (m/v)