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

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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)

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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.

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1. Mass per Volume Concentration Example #1 – 5.674 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 = 5.674 g/0.250L C = 22.7 g/L Step Two: ○ 5.674 g x (1 mole/106 g) = 0.0532 mol ○ 0.0532 mol/0.250 L = 0.214 M

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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

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

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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).

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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)

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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.

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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)

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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 )

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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 1 000 000 mL of well water, there are 20 g Fe. convert ppm Fe into grams Fe. 20 ppm = 20. g Fe/1 000 000 mL H 2 O = 0.020 g/L Step Two: Convert g/L Fe into M: 0.020 g/L Fe x (1 mole Fe/55.847 g Fe) = 0.000358 M

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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)

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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 )

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

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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 10 -6 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 10 -6 M HgCl 2, there will be 6.08 x 10 -6 M Hg +2 ions. Step Two: Convert from M to g/L: 6.08 x 10 -6 mol/L x 200.59 g/mol = 1.22 x 10 -3 g/L Step Three: Convert from g/L to ppm: 1.22 x 10 -3 g/1000 mL x (1 x 10 6 )= 1.22 ppm Step Four: Convert from g/L to ppb: 1.22 x 10 -3 g/1000 mL (1x 10 9 )= 1220 ppb Step Five: To determine % (m/v): 1.22 x 10 -3 g/1000 mL x 100 % = 0.000122 % (m/v)

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