Hydrology Project Training Module File: “ 02 Basic chemistry concepts.doc” … This module introduces basic concepts of chemistry required by chemists at all …

This module introduces basic concepts of chemistry required by chemists at all levels in their daily work in the laboratory. No prior training in other module is needed to complete this module successfully.

Basic chemistry concepts

Units of measurement
Elements, compounds and molecular weights
Equivalent weights and chemical reactions
Titrimetric calculations
Significant figures

1. Units of measurement: concentration units
Example:
Four kg common salt is thrown in a tank containing 800 m3 of water. What is the resulting concentration of salt in mg / L?

2. Elements, compounds & molecular weights
• Elements combine to make compounds which do not have any net charge
• Compounds dissolved in water dissociate into charged ions
• Radicals are groupings of elements acting together as charged ions

Example:

Write the molecular formula for aluminium sulphate (alum) if each molecule has 18 molecules of water of crystallisation.

No. of +ive charges on 2Al3+ = 6
No. of -ive charges on 3SO42- = 6

The formula is Al2(SO4)3.18H2O

3. Equivalent weight
• Molecular weight / Valency
• Valency is equal to:
- absolute number of ion charge
- number of H+ or OH- ions that can combine with the ion
- absolute number of change in charge of ion in a reaction
• Quantity of chemicals equivalent to each other
• One chemical expressed as another
• Same number of equivalents of reactants in a chemical reaction

Example: Express 120 mg Ca++ / L as mg CaCO3/L

- Equivalent weight of Ca++ = 20
- Equivalent weight of CaCO3 = 50

4. Titrimetric method
• Standard solutions contain known concentration of one reactant
• 1 N solution contains 1 eq wt/L
• React standard solution against unknown concentration in sample
• End point is determined using indicator
• Eq of reactant in standard = Eq of reactant in sample

Download Basic Concepts Of Chemistry.pdf