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Chemistry 30 – Unit 2 – Solubility – Ch. 16 in Text. 05 – Other measure s of concentration. More than just Molarity ….
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Chemistry 30 – Unit 2 – Solubility – Ch. 16 in Text 05 – Other measure s of concentration
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 M1V1 = M2V2, C1V1 = C2V2.
1. Mass per Volume Concentration • Example #1 – 5.674 g of Na2CO3 is dissolved in 250 mL of water. What is the concentration in g/L? What is the molar concentration? • Step One: C = m/v C = 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/0.0250 L = 0.214 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 x106) • ppm (v/v) = [volume of solute (mL) / volume of solution (mL)] x (1 x106) • ppm (m/m) = [mass of solute (g) / mass of solution (g)] x (1 x106)
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 H2O = 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
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 x109) • ppb (v/v) = [ volume of solute (mL) / volume of solution (mL)] x (1 x109) • ppb (m/m) = [mass of solute (g) / mass of solution (g)] x (1 x109)
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 10-6 M HgCl2? • Step One: Write and balance the dissociation formula: HgCl2(s) Hg+2(aq) + 2Cl-(aq) • The molar ratio between HgCl2(s) and Hg+2(aq) tell us that for every 6.08 x 10-6 M HgCl2, 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 106)= 1.22 ppm • Step Four: Convert from g/L to ppb: 1.22 x 10-3 g/1000 mL (1x 109)= 1220 ppb • Step Five: To determine % (m/v): 1.22 x 10-3 g/1000 mL x 100 % = 0.000122 % (m/v)