# Solution Stoichiometry

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. • Solution Stoichiometry - Calculate Moles or Mass:
• Solution Stoichiometry - Determine Volume:
• Solution Stoichiometry - Determine Molarity:

## Titrations

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.

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.

Indicator:
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.

#### Homework from 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