(General Chemistry 2) Concentration Units and Comparison of Concentration Units
Updated: May 15, 2021
Concentration Units
•In chemistry, concentration refers to the amount of a substance per defined space.
•Concentration is the ratio of solute in a solution to either solvent or total solution
Parts per million (ppm)
(i) Formula: parts per million = grams of solute / grams of solution x 10^6
(ii) Unit: unitless
(iii)Problem:
What is the concentration, in ppm, if 0.808 g of CaCl2 is dissolved in 250.0 mL of water?
(iv)Solution:
parts per million = 0.808 𝑔 solute /
0.808 𝑔 CaCl2+250 𝑔 𝐻2𝑂 x 10^6
parts per million = 0.0032216 x 10^6
parts per million = 3,221.60 ppm
Parts per billion (ppb)
(i) Formula: parts per million = grams of solute / grams of solution x 10^9
(ii) Unit: unitless
(iii)Problem:
What is the concentration, in ppb, of polychlorinated biphenyls (PCBs) in a chemical spill, if there is 0.060 mg in 4,600 kg of soil?
(iv)Solution:
1. Concert first the solute to g from mg, and the solution from kg to g.
0.60 mg x 1 g / 1000 mg = 0.000060 g PCB
4600 kg x 1000 g / 1 kg = 4 600 000 g soil
2. Then compute for the ppb from the mass of solute and mass of solution.
parts per billion = 0.00060 g solute / 4 600 000 soil x 10^9
parts per billion = 0.00000000001304347 x 10^9
parts per billion = 0.013 ppb
parts per billion = 0.013 ppb
Mole Fraction (X)
(i) Formula: mole fraction of component A = mole of A / sum of moles of all componets
(ii) Unit: unitless
(iii)Problem:
0.100 mole of NaCl is dissolved in 100.0 grams of pure H2O. What is the mole fraction of NaCl?
(iv)Solution:
1. Calculate first the number of moles of H2O.
100 g / 18 g.mol = 5.56 mol of H2O
2. Add that value to the 0.100 mol of NaCl.
= 5.56 mol H2O + 0.100 mol NaCl = 5.66 mol
3. Compute for the mole fraction of NaCl.
Mole fraction NaCl = 0.100 mol / 5.66 mol = 0.018
4. Compute for the mole fraction of H2O.
Mole fraction H2O = 5.56 mol H20 / 5.66 mol NaCl = 0.982
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