Key Points

In a chemical reaction, the total mass of the substances that react (reactants) is equal to the total mass of the substances formed (products). This can be expressed as: $\sum \text{mass}_{\text{reactants}} = \sum \text{mass}_{\text{products}}$.

In a pure chemical compound, the elements are always present in a definite proportion by mass. For example, in water ($H_2O$), the mass ratio of hydrogen to oxygen is always $1:8$.

To find the mass of one reactant needed to completely react with another, use their fixed mass ratio. If element A and B combine in a mass ratio of $a:b$, then for a mass $m_A$ of A, the required mass of B is $m_B = \frac{b}{a} \times m_A$.

A key concept from Dalton's atomic theory is that atoms combine in the ratio of small whole numbers to form compounds. This explains why the ratio of atoms in a molecule like water ($H_2O$) is a simple $2:1$.

The atomic mass unit (u) is the standard unit for indicating mass on an atomic or molecular scale. It is defined as a mass exactly equal to one-twelfth the mass of one atom of carbon-12, so $1 \text{ u} = \frac{1}{12} \times \text{mass of one C-12 atom}$.

The molecular mass of a substance is the sum of the atomic masses of all the atoms in a single molecule of that substance. For a molecule with formula $A_x B_y$, the molecular mass is $(x \times \text{atomic mass of A}) + (y \times \text{atomic mass of B})$.

To calculate the molecular mass of nitric acid ($HNO_3$), sum the atomic masses: $1 \times (\text{atomic mass of H}) + 1 \times (\text{atomic mass of N}) + 3 \times (\text{atomic mass of O})$. This gives $1 + 14 + 3(16) = 63 \text{ u}$.

Formula unit mass is calculated in the same way as molecular mass but is used for substances whose constituent particles are ions, like ionic compounds. It is the sum of the atomic masses of all atoms in a formula unit.

To find the formula unit mass of calcium chloride ($CaCl_2$), sum the atomic masses of its ions: $(1 \times \text{atomic mass of Ca}) + (2 \times \text{atomic mass of Cl})$. This gives $40 + 2(35.5) = 111 \text{ u}$.

To find the simplest whole number ratio of atoms in a compound, divide the mass of each element by its atomic mass. The resulting values represent the ratio of the number of atoms.

In water, the mass ratio of Hydrogen to Oxygen is $1:8$. Given atomic masses H=1u and O=16u, the ratio of atoms is $(\frac{1}{1}) : (\frac{8}{16}) = 1 : 0.5$. To get a whole number ratio, multiply by 2 to get $2:1$.

Valency is the combining capacity of an element. The 'criss-cross' method uses valencies to determine the chemical formula. If element A has valency $x$ and element B has valency $y$, the formula is $A_y B_x$.

When a compound contains a polyatomic ion, and more than one unit of that ion is needed, the ion's formula is enclosed in parentheses with a subscript. For example, in Calcium Hydroxide, one $Ca^{2+}$ ion combines with two $OH^{-}$ ions to form $Ca(OH)_2$.

Atomicity is the number of atoms constituting a molecule. For example, oxygen gas ($O_2$) is diatomic (atomicity 2), while ozone ($O_3$) is triatomic (atomicity 3).