Language of Chemistry

A. Chemical Symbols:

An abbreviation representing one atom of a specific element. Derived from:

• First letter of English name: $\\text{H}$ (Hydrogen), $\\text{C}$ (Carbon), $\\text{O}$ (Oxygen).
• First two letters: $\\text{He}$ (Helium), $\\text{Al}$ (Aluminium), $\\text{Ca}$ (Calcium).
• Latin names: Sodium $\\to \\mathbf{\\text{Na}}$ (Natrium), Potassium $\\to \\mathbf{\\text{K}}$ (Kalium), Iron $\\to \\mathbf{\\text{Fe}}$ (Ferrum), Copper $\\to \\mathbf{\\text{Cu}}$ (Cuprum), Gold $\\to \\mathbf{\\text{Au}}$ (Aurum), Silver $\\to \\mathbf{\\text{Ag}}$ (Argentum), Lead $\\to \\mathbf{\\text{Pb}}$ (Plumbum).

B. Valency & Variable Valency:

The combining capacity of an atom of an element, determined by the number of valence electrons lost, gained, or shared to achieve a stable octet (or duplet).

• Monovalent (1): $\\text{Na}^+, \\text{K}^+, \\text{Cl}^-, \\text{H}^+$.
• Divalent (2): $\\text{Mg}^{2+}, \\text{Ca}^{2+}, \\text{O}^{2-}, \\text{S}^{2-}$.
• Trivalent (3): $\\text{Al}^{3+}, \\text{N}^{3-}, \\text{P}^{3-}$.
• Tetravalent (4): $\\text{C}^{4+}, \\text{Si}^{4+}$.
• Variable Valency: Transition elements exhibit more than one valency due to participation of inner $d$-electrons: $$\\text{Iron: } \\text{Fe}^{2+} \\text{ (Ferrous / Iron(II))} \\quad \\text{and} \\quad \\text{Fe}^{3+} \\text{ (Ferric / Iron(III))}$$ $$\\text{Copper: } \\text{Cu}^+ \\text{ (Cuprous / Copper(I))} \\quad \\text{and} \\quad \\text{Cu}^{2+} \\text{ (Cupric / Copper(II))}$$

The Criss-Cross Method:

1. Write the basic radical (cation) on the left and the acidic radical (anion) on the right with their respective valencies as superscripts: $\\text{A}^x \\quad \\text{B}^y$.
2. Divide the valencies by their highest common factor (if any) to get the simplest integer ratio.
3. Interchange (criss-cross) the numbers and write them as subscripts to the opposite symbols: $\\text{A}_y\\text{B}_x$. (If a radical is polyatomic, enclose it in brackets before adding a subscript $> 1$).

Illustrative Examples:

• Aluminium Sulphate: Cation $\\text{Al}^{3+}$, Anion $\\text{SO}_4^{2-}$. Criss-cross valencies $3$ and $2$ $\\implies \\mathbf{\\text{Al}_2(\\text{SO}_4)_3}$.
• Calcium Carbonate: $\\text{Ca}^{2+}$ and $\\text{CO}_3^{2-}$. Ratio $2 : 2 = 1 : 1 \\implies \\mathbf{\\text{CaCO}_3}$.
• Ammonium Phosphate: $\\text{NH}_4^+$ and $\\text{PO}_4^{3-} \\implies \\mathbf{(\\text{NH}_4)_3\\text{PO}_4}$.

A. Balancing Chemical Equations:

By the Law of Conservation of Mass, the number of atoms of each element on the Reactants side must equal the number of atoms on the Products side.

B. Relative Molecular Mass (RMM) & Percentage Composition:

• RMM: Sum of the atomic masses of all atoms present in one molecule of the compound (expressed in atomic mass units $\\text{amu}$ or dimensionless).
• Percentage Composition by Mass: $$\\mathbf{\\% \\text{ of Element} = \\frac{\\text{Total mass of that element in 1 molecule}}{\\text{Relative Molecular Mass of the compound}} \\times 100\\%}$$

Problem 1: Calculate the Relative Molecular Mass (RMM) of hydrated copper sulphate $\\text{CuSO}_4\\cdot 5\\text{H}_2\\text{O}$. (Given atomic masses: $\\text{Cu} = 63.5, \\text{S} = 32, \\text{O} = 16, \\text{H} = 1$).

Solution:

Problem 2: Calculate the percentage of nitrogen in urea $[\\text{CO}(\\text{NH}_2)_2]$. (Given: $\\text{C} = 12, \\text{O} = 16, \\text{N} = 14, \\text{H} = 1$).

Solution:

1. Calculate RMM of urea $\\text{CH}_4\\text{N}_2\\text{O}$:

2. Mass of nitrogen in one molecule $= 2 \\times 14 = 28$.

3. Percentage of Nitrogen: