Chapter 3 Amount of substance Flashcards
Amount of substance (n)
Used to count the number of particles in a substance
Units of amount of substance
Mole (mol.)
Avogadro Constant (N)
6.02 x 10^23
- The number of particles in 1 mole of carbon-12 or of a substance
Mole
Amount of a substance that contains 6.02 x 10^23 particles
Relative atomic mass or relative molar mass
Mass (in grams) of 1 mole of atoms of an element
Chemical formulae
- Molecular formulae
- Empirical formulae
Chemical formulae: Molecular formulae
The number of atoms of each element in each molecule
Chemical formulae: Empirical formulae
The simplest whole-number ratio of atoms of each element in a compound
Relative molecular mass
Compares the mass of a molecule with the mass of an atom of carbon-12
Relative formula mass
Compares the mass of a formula unit with the mass of an atom of carbon-12
Analysis
Investigating the chemical composition of a substance
Water of crystallisation in hydrated salts
When water molecules are part of their crystalline structure
Assumptions made when finding the formula of a hydrated salt
- All water has been lost. A good solution is to heat to constant mass
- No further decomposition (some salts decompose further when heated (e.g. copper sulfate -> copper oxide)
Standard solution
A solution of known concentration
Molar gas volume
The volume per mole of gas molecules at a stated temperature and pressure (normally RTP)
RTP
Room temperature and pressure
- 20 degrees celsius
- 101 kPa (1atm) pressure
What volume does 1 mole of gas molecules at RTP take up?
24 dm3
The ideal gas equation
pV = nRT
Assumptions made for the molecules in an ideal gas:
- Random motion
- Elastic collisions
- Negligible size
- No intermolecular forces
Stoichiometry
The balancing numbers which gives the ratio of the moles in each substance
Why do chemists balance equations
To find:
- The quantities of reactants required to prepare a required quantity of a product
- The quantities of products that should be formed from certain quantities of reactants
Percentage yield
Actual yield/theoretical yield x 100
Theoretical yield
The maximum possible amount of product
Why is theoretical yield difficult to achieve?
- The reaction may not have gone to completion
- Other reactions may have taken place alongside the main reaction
- Purification of the product may result in loss of some product
Actual yield
The amount of product obtained from the reaction
- Usually lower than theoretical yield
The limiting reagent
The reactant that is not in excess which woll be completely used up first and stop in reaction
Atom economy
A chemical reaction is a measure of how well atoms have been utilised
Characteristivs of reactions with high atom economies
- Produce a large proportion of desired products and few unwanted waste products
- are important for sustainability as they make the best use of natural resources
Atom economy equation
sum of molar masses of desired products/ sum of molar masses of all products
x 100
What does efficiency depend on
- Atom economy
- Percentage yield
e.g. a reaction could have a high atom economy but low percentage yield
