Now let's use molarity in some stoichiometric calculations.
Calculate the minimum amount (in liters) of a 0.050 M BaCl2 solution that is required to precipitate all the SO42-(aq) in 0.10 liter of a 0.10 M Na2SO4 solution.
Ba2+(aq) + SO42-(aq) → BaSO4
In order to use the chemical equation we need to work in terms of moles. The concentration of SO42-(aq) is converted into moles by multiplying by the volume.
A useful way to determine a solute's concentration in a solution is to react the solution with a solute in another solution of known concentration. This is known as a Titration.
- Experiment which determines the concentration of a solute (reactant) using its reaction of known stoichiometry with another solution (reactant) of known concentration.
For example, if I have a solution of sulfuric acid, H2SO4 (aq), but don't know its concentration, then I can react it with a NaOH solution of known concentration.
2 NaOH(aq) + H2SO4 (aq) → Na2SO4 (aq) + 2 H2O(l)
In a titration the titrant is added dropwise until the reaction is complete.
- Equivalence Point:
- Point at which stoichiometrically equivalent quantities are brought together.
40.0 mL of 0.20 M NaOH is needed to neutralize (reach equivalence point) for 20.0 mL of H2SO4 solution. What is the concentration of H2SO4 solution?
2 NaOH + H2SO4 → Na2SO4 + 2 H2O
How do we know when the reaction is complete? We add a tiny amount of indicator to the analyte that will change color when the solution has excess titrant (e.g. excess OH-). For example, phenolphthalein molecules are colorless in neutral and acidic solutions, but are reddish purple in basic (i.e., excess OH-) solutions.
- A material which (by changing colors or other means) signals that the equivalence point has been attained.
- End Point:
- When the indicator indicates that the equivalence point have been reached (e.g., changes color).
Indicators need to be chosen carefully, so they don't change color too soon or too late.
Chemisty, The Central Science, 10th Ed.
4.29, 4.31, 4.59, 4.61, 4.63, 4.65, 4.67 4.69, 4.71, 4.73, 4.77, 4.79, 4.81, 4.83, 4.85