Chemical Formulae

1. Relative atomic mass, Ar is the atomic mass of an atom when compared to a standard atom
2. Standard atom:

Carbon scale: standard atom of comparison internationally.

• a carbon-12 atom is 12 times heavier than an atom of hydrogen,
• used as the reference standard in mass spectrometers,
• exists as a solid at room temperature,
• most abundant carbon isotope, happening about 98.89% and
• carbon-12 is close to the agreement based on oxygen.

3. Relative molecular mass, Mr of a substances is the average mass of a molecule (two or more atoms) of the substances when compared 1/12 with of the mass of a carbon-12 atom.
4. Relative formula mass, Fr is for ionic compound which is calculated by adding up the relative atomic masses of all the atoms.

5. Avogadro constant / Avogadro’s number is 6.02 x 10²³
6. Atomic substances

• Elements – all the particles are atoms.
• Example: zinc (Zn), sodium (Na), aluminium (Al) and all noble gases, argon (Ar), helium (He) and neon (Ne).
• RAM (Relative Atomic Mass) of  Na = 23

7. Molecular substances

• Covalent compounds – the particles are molecules.
• Example: carbon dioxide (CO₂), water (H₂O) and non-metal elements, iodine (I₂), nitrogen (N₂) and oxygen (O₂).
• RMM (Relative Molecular Mass) of I₂ = 127 + 127 = 254

8. Ionic substances

• Ionic compounds – the particles are ions.
• Example: sodium chloride (NaCl), hydrochloric acid (HCl) and potassium iodide (KI).
• RFM (Relative Formula Mass) of HCl = 1 + 35.5 = 36.5

9. Avogadro’s Law / Gas Law states that equal volumes of all gases contain the same number of molecules under the same temperature and pressure.

• Example: equal volumes of molecular hydrogen and nitrogen would contain the same number of molecules under the same temperature and pressure.

10. Volume of gas (dm³) = Number of moles of gas x Molar volume
11. Room temperature and pressure (r.t.p.) = 24 dm³ mol^-1 (25°C and 1 atm)