Stoichiometry

Question 1

State the formulae of the elements and compounds named in the subject content

Answer 1

Elements: Symbols representing individual elements, such as H for hydrogen, O for oxygen, Na for sodium.
Compounds: Combinations of elements represented by chemical formulas, such as H2O for water, NaCl for sodium chloride, CO2 for carbon dioxide.

Question 2

Define the molecular formula of a compound as
the number and type of different atoms in one
molecule

Answer 2

The molecular formula of a compound shows the number and type of different atoms present in one molecule of the compound. For example, the molecular formula of water (H2O) indicates that each water molecule contains two hydrogen atoms and one oxygen atom.

Question 3

Deduce the formula of a simple compound from
the relative numbers of atoms present in a model
or a diagrammatic representation

Answer 3

The formula of a simple compound can be deduced from the relative numbers of atoms present in a model or a diagrammatic representation. For example, if a model shows two hydrogen atoms bonding with one oxygen atom, the formula would be H2O.

Question 4

Construct word equations and symbol equations
to show how reactants form products, including
state symbols

Answer 4

Word equations describe chemical reactions using words, while symbol equations represent reactions using chemical symbols and formulas. For example, the word equation “hydrogen + oxygen → water” can be represented as the symbol equation “2H2 + O2 → 2H2O”. State symbols (s for solid, l for liquid, g for gas, aq for aqueous solution) indicate the physical state of the reactants and products.

Question 5

Define the empirical formula of a compound as
the simplest whole number ratio of the different
atoms or ions in a compound

Answer 5

The empirical formula of a compound represents the simplest whole-number ratio of the different atoms or ions in the compound. For example, the empirical formula of hydrogen peroxide is H2O2, but its molecular formula is H2O2.

Question 6

Deduce the formula of an ionic compound from
the relative numbers of the ions present in a
model or a diagrammatic representation or from
the charges on the ions

Answer 6

The formula of an ionic compound can be deduced from the relative numbers of ions present in a model or a diagrammatic representation, or from the charges on the ions. For example, sodium chloride (NaCl) consists of one sodium ion (Na+) and one chloride ion (Cl-).

Question 7

Construct symbol equations with state symbols,
including ionic equations

Answer 7

Symbol equations with state symbols include information about the physical state of reactants and products. For example, the ionic equation for the reaction between silver nitrate solution (aq) and sodium chloride solution (aq) to form silver chloride precipitate (s) and sodium nitrate solution (aq) is written as Ag+(aq) + Cl-(aq) → AgCl(s) + Na+(aq) + NO3-(aq).

Question 8

Deduce the symbol equation with state
symbols for a chemical reaction, given relevant
information

Answer 8

The symbol equation with state symbols for a chemical reaction can be deduced given relevant information about the reactants and products, including their physical states. For example, the burning of methane gas (CH4) in oxygen gas (O2) to form carbon dioxide gas (CO2) and water vapor (H2O) can be represented by the equation CH4(g) + 2O2(g) → CO2(g) + 2H2O(g).

Question 9

Describe relative atomic mass, Ar\, as the average
mass of the isotopes of an element compared to
1/12th of the mass of an atom of 12C

Answer 9

Relative atomic mass, denoted as Ar, is the average mass of the isotopes of an element relative to 1/12th of the mass of an atom of carbon-12 (12C).
It is a weighted average of the masses of all isotopes of an element, taking into account their natural abundances.
For example, the relative atomic mass of carbon is approximately 12.01, meaning that on average, a carbon atom is 12.01 times heavier than 1/12th of the mass of a carbon-12 atom.

Question 10

Define relative molecular mass, Mr
, as the sum
of the relative atomic masses. Relative formula
mass, Mr
, will be used for ionic compounds

Answer 10

Relative molecular mass, denoted as Mr, is the sum of the relative atomic masses of all the atoms in a molecule.
It represents the mass of a molecule relative to 1/12th of the mass of an atom of carbon-12 (12C).
For example, the relative molecular mass of water (H2O) is approximately 18.02, calculated by adding the relative atomic masses of two hydrogen atoms (1.01 each) and one oxygen atom (16.00).

Question 11

Calculate reacting masses in simple proportions.
Calculations will not involve the mole concept

Answer 11

Reacting masses can be calculated in simple proportions based on the stoichiometry of a chemical reaction.
This involves determining the mass of reactants needed to completely react with each other based on their balanced chemical equation.
For example, in the reaction between hydrogen gas (H2) and oxygen gas (O2) to form water (H2O), the reacting masses can be calculated based on the stoichiometry of the reaction without involving the mole concept. If the balanced equation is 2H2 + O2 → 2H2O, then 2 grams of hydrogen gas will react with 16 grams of oxygen gas to produce 36 grams of water.

Question 12

State that

Answer 12

Concentration can be measured in
g /dm3
or mol/dm3

Question 13

Mole

Answer 13

mol is the unit of amount of
substance and that one mole contains
6.02 × 1023 particles, e.g. atoms, ions, molecules;
this number is the Avogadro constant