After few weeks of learning chemistry, there are still some students who wrote in to Berry Berry Easy to express their problems in mastering chemical formulae and chemical equations. Some ask if they could just forget about it and catch up later. The answer is NO. Among basic skills which students must master to understand chemistry, none is more important than mastering chemical formulae and chemical equations. Most newcomers to the subject of chemistry are scared off by the rather complex chemical formulae and equations, although they become easier when you understand the underlying concept behind them.
Chemical formulae are simply there to describe chemical reactions as denoted by the chemical equations. Confusing? Nay, it should be. Some memorising and practise will go a long way in understanding this topic.
If you think this is hard, try understanding the difficulty of chemistry students before Mr Berzelius devised the current chemical formulae writing system. So give it a try before saying that this is hard. For those who do not find this difficult, congratulation. But do work hard in learning how to express information in a chemical formula properly. So Berry Berry Teacher would like to share with all some notes as shown below.
SPM Form 4 – Terminology and Concepts: Chemical Formulae and Equations – Part 1
1. Relative atomic mass, Ar is the atomic mass of an atom when compared to a standard atom
2. Standard atom:
Hydrogen scale: hydrogen is the lightest atom of all and the mass of one hydrogen atom was assigned 1 unit.
Weakness of Hydrogen scale:
- not too many elements can react readily with hydrogen,
- the reactive masses of some elements were not accurate,
- hydrogen exists as a gas at room temperature and
- has a number of isotopes with different masses.
Helium scale: the second lightest atom of all and the mass of one helium atom was assigned 1 unit.
Weakness of Helium scale:
- Mass of 1 helium atom = 4 times the mass of a hydrogen atom
- So, mass of 1 helium atom = 4 times 1/12 mass of a carbon atom
- helium exists as a gas at room temperature and
- helium is an inert gas.
Oxygen scale: chose as the standard atom to compare the masses of atoms
Weakness of Oxygen scale:
- the existence of three isotopes of oxygen were discovered,
- natural oxygen (containing all the three isotopes) as the standard (Chemist) and
- used the isotopes oxygen-16 as the standard (Physicists).
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.
- Relative atomic mass, Ar of helium = 4
- Relative molecular mass, Mr of CO2 = 12 + 2(16) = 44
- Relative formula mass, Fr of NaCl = 23 + 35.5 = 58.5
- Relative formula mass, Na2CO3·10H2O = 2(23) + 12 + 3(16) + 10 [2(1) + 16] = 286
Try to solve some of the examples without looking at the answers. If you can understand this, then stay tune and log in again for Part 2 of this topic’s notes. If you cannot understand the examples, try and try and try and try and try again until you are good with it. Till then.