5.3 Quantitative Chemistry (5.3.2 Use of Amount of Substance in Relation…
5.3 Quantitative Chemistry
5.3.1 Chemical Measurements, Conservation of Mass and the Quantitive Interpretation of Chemical Equations
220.127.116.11 Conservation of Mass and Balanced Chemical Equations
The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of reactants.
Chemical reactions can be represented by symbol equations which are balanced in terms of the number of atoms of each element involved on both sides of the equation.
18.104.22.168 Relative Formula Mass
The RFM of a compound is the sum of the ARAM of the atoms in the formula.
In a balanced chemical equation, the sum of the RFM of the reactants is equal to the sum of RFM in the products.
22.214.171.124 Mass Changes When a Reactant or Product is a Gas
Some reactants may appear to involve a change in mass.
A change in mass can usually be explained because a reactant or product is a gas and its mass has not been taken into account.
126.96.36.199 Chemical Measurements
Whenever a measurement is made there is always some uncertainty about the result obtained.
5.3.2 Use of Amount of Substance in Relation to Masses of Pure Substances
Chemical amounts are measured in moles.
The symbol for moles is mol.
The mass of one mole of a substance in grams in numerically equal to its relative formula mass.
The number of atoms, molecules, or ions in a mole of a given substance is the Avogadro constant (6.02 x 10^23)
188.8.131.52 Amounts of Substances in Equations
The masses of products and reactants can be calculated from balanced symbol equations.
Chemical equations can be interpreted in terms of moles.
184.108.40.206 Using Moles to Balance Equations
The balancing numbers in symbol equations can be calculated from the mass of reactants and products. Convert the masses in grams into amounts in moles. Convert the moles into simple whole number ratios.
220.127.116.11 Limiting Reactants
In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used.
The reactant that is completely used up is called the limiting reactant because it limits the amount of products.
18.104.22.168 Concentration of Solutions
Many chemical reactions take place in solution.
The concentration of a solution can be measured in mass per given volume, eg. grams per dm^2.
Chemists use quantitative analysis to determine the formulae of compounds and the equations for reactions. Given this information, analysts can then use quantitative methods to determine the purity of chemical samples and to monitor the yield from chemical reactions. Chemical reactions can be classi ed in various ways. Identifying different types of chemical reaction allows chemists to make sense of how different chemicals react together, to establish patterns and to make predictions about the behaviour of other chemicals. Chemical equations provide a means of representing chemical reactions and are a key way for chemists to communicate chemical ideas.