Chemistry: Acid-Base Titrations

Study Notes

Standard solutions are solutions with a known concentration and composition.
Primary Standard solutions have a precisely known concentration and are stable over time.
- They are prepared by dissolving a precisely weighed amount of a primary standard substance in a volumetric flask.
Requirements for a primary standard substance:
- High purity and known composition.
- Stable and resistant to reactions with air.
- Not deliquescent or hygroscopic, and nonvolatile.
- Can be dried at 105-110°C without decomposition.
- High molecular weight to minimise weighing errors.
- Reacts quantitatively with other substances.
Examples of primary standard substances:
- Acids: Potassium acid phthalate, Oxalic acid, Benzoic acid.
- Bases: Sodium carbonate, Borax (sodium borate).
Secondary Standard solutions have an approximately known concentration.
- Their exact concentration is determined by a process called standardization.
Standardization involves:
- Titrating the secondary standard against a primary standard solution or a previously standardized secondary standard.
Examples of secondary standards:
- HCl and NaOH.

The most common methods for expressing the concentration of solutions are:
- Molarity (M) - The number of moles of solute per liter of solution.
- Normality (N) – The number of equivalent weights of solute per liter of solution.

Equation:

 M = Number of moles of solute / Volume in L of solution

For molecules: The mole is the gram-molecular weight (molecular weight expressed in grams).
For atoms: The mole is the gram-atomic weight (atomic weight expressed in grams).
Example:
- The molecular weight of NaOH is 40, therefore 1 mole of NaOH weighs 40 grams.
- The atomic weight of Na is 23, therefore 1 mole of Na weighs 23 grams.

Equation: N = Number of equivalent weights of solute / Volume in L of solution
Example: 2 N HCl means 2 equivalent weights of HCl per 1 L of the solution.

It is the weight of a substance that is equivalent in reactivity to 1 mole of hydrogen.
The exact definition and calculation of equivalent weight vary depending on the type of reaction taking place.