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Wize University Chemistry Textbook > Solutions and Colligative Properties

Solution Stoichiometry

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Everything You Need to Know About Concentrations


Solutions

  • Homogeneous mixture containing a solvent (liquid) and one or more solutes (can be gas, liquid or solid)
  • Concentrations can be measured in different ways:
  1. Molarity (M): moles of solutelitre of solution=molL\dfrac{\text{moles of solute}}{\text{litre of solution}}=\dfrac{mol}{L}
  2. Mass Percent (%w/w): g of soluteg solution×100\cfrac{\text{g of solute}}{\text{g solution}}\times100%
  3. Percent weight by volume (%w/v): g of solute100mL of solution×100\cfrac{\text{g of solute}}{\text{100mL of solution}}\times100%
  4. Percent volume by volume (%v/v): mL of solute100mL of solution×100\cfrac{\text{mL of solute}}{\text{100mL of solution}}\times100% %w/v=\cfrac{\text{g of solute}{{\text{100 mL of solution}} \times 100%
  5. Concentration can be expressed in parts per million (ppm) or parts per billion (ppb), usually when working with trace impurities (ex: the label on bottled water)
[mass of solute (g)]mass of solution (g)x106 (for ppm) or 109 (for ppb)\frac{\left [mass\ of\ solute\ \left(g\right)\right]}{mass\ of\ solution\ \left(g\right)}x10^6\ \left(for\ ppm\right)\ or\ 10^9\ \left(for\ ppb\right)
  • To convert from mL of a liquid to grams, use the density.
volume (mL) ⋅ density (gmL) =mass (g)volume\ \left(mL\right)\ \cdot\ density\ \left(\frac{g}{mL}\right)\ =mass\ \left(g\right)

Dilutions

  • When diluting a solution from an initial concentration, we can find the final volume for a desired final concentration or vice versa:
  • The number of moles of solute does not change, but the volume of the solvent increases.

n1=n2,thereforen_1=n_2 , therefore

C1V1=C2V2C_1V_1 = C_2V_2

Note that a diluted/standard/and stock solution are all different!

Standard solution: solution that we know the concentration for accurately
Stock solution: standard solutions kept in concentrated form
Dilution: Stock and standard solutions are diluted to prepare solutions of a lower concentration
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53.4 mL of a 1.50 M solution of NaCl is on hand, but you need some 0.800 M solution. How many mL of 0.800 M can you make?

Using the dilution equation, we write:

C1V1=C2V2C_1V_1=C_2V_2
(1.50 mol/L) (53.4 mL) = (0.800 mol/L) ( X mL)
x = 100 mL
Notice that the volumes need not be converted to liters, as long as volume units are the same on both sides.
What is the molarity (M) of potassium ions in a solution prepared by mixing 100 mL of 0.250 M potassium bicarbonate with 400 mL of 0.400 potassium sulfate?
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How many carbon atoms are in 10.00 mL of ethanol ( CH3CH2OH)?
The density of ethanol is 0.789 g/mL.