2.1.7 Types of Formulae

Question 1

What is the difference between empirical formula and molecular formula?

Answer 1

A molecular formula is the number and type of atoms of each element present in a molecule; whereas empirical formula is the simplest whole number ratio of atoms of each element present in a compound - it is the ratio of elements there rather than the actual number, which is what molecular formula shows.

Question 2

What are empirical formulae used for and why?

Answer 2

1. Ionic compounds;
2. Giant covalent compounds;
   Because: these structures are giant (contain millions of atoms), so the easiest way to show the formula is in the form of a ratio - empirical formula.

Question 3

How can one find the empirical formula for magnesium oxide via experimental means?

Answer 3

1. Measure the mass of Mg before the reaction;
2. Heat Mg and O in a crucible;
3. Wait for crucible to cool;
4. Measure mass of magnesium oxide formed;
5. Use the formula moles=mass/molar mass - to find the no. of moles of Mg/O reacted;
6. Determine empirical formula for mag. oxide;

Question 4

Name the process that one would use to find the empirical formula of a substance.

Answer 4

1. Divide the amount of each element present by its molar mass (RAM) - to give ‘molar ratio.’ This is using the equation moles=mass/molar mass;
2. Divide the answer for each element by the smallest number - to give ratio in the form 1:x;
3. Name any ratios whole numbers: 1:1.5 would go to 2:3;

Question 5

When would one not use empirical formula?

Answer 5

With simple molecules, they would just use molecular formula.

Question 6

What is special about propane? Use this to explain why chemists rarely use empirical formulae for smaller molecules.

Answer 6

Propane is C3H8; its molecular formula is the same as its empirical formula - this is mostly true of smaller atoms, which is why chemists use molecular formula for these and not empirical formula.

Question 7

What is a crucial aspect of chemistry, which molecular formula fails to show?

Answer 7

Molecular formula fails to show the order in which the atoms are bonded to each other.

Question 8

How should one calculate molecular formulae? Eg: a compound has an empirical formula of CH2, and an Mr of 56, what is its molecular formula?
Generalize this process.

Answer 8

1. Empirical formulae:
   Through the molar masses (RAMs, g/mol) and relative molecular masses of compounds through exps;
2. Empirical f mass of CH2 = 12+(1x2)=14;
3. No of CH2 units in a molecule= 56/14=4;
4. Molecular formula: (4*CH2)=C4H8 - butene: an alkene.
   General process:
5. Find the f mass of the empirical formula;
6. Find the no of molecules, by dividing the no the question gives you by the one you just worked out;
7. Times the no. of molecules you have, by the empirical formula for the substance in the question to get molecular formula.

Question 9

What is special about the molecular mass of a compound?

Answer 9

It must always be the same as the mass of the empirical formula, or a whole number multiple of it.

Question 10

Define ‘Mr.’

Answer 10

1. Molecular mass;

2. RAMs of elements added together;