C3 - Quantative Chemistry

Topic C3 - Quantative Chemistry

Relative Formula Mass

Percentage mass of an element in a compound =

Ar x number of atoms in that element / Mr of the compound x 100

The Mole 

Avogadro constant = 6.02 x 10^23

One mole of any substance is just an amount of that substance that contains an Avogrado number of particles. The particles could be atoms, molecules, ions or electrons.

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass (Ar or Mr) for that substance.

e.g.

Carbon has an Ar of 12, so one mole of carbon weighs exactly 12g

Nitrogen gas (N2) has an Mr of 28 (2x14), so one mole of N2 weighs exactly 28g

Carbon dioxide (CO2) has an Mr of 44 (12 + [2 x 16]), so one mole of CO2 weighs exactly 44g.

This means that 12g of Carbon, or 28g of N2, or 44g of CO2, all contain the same number of particles, namely one mole or 6.023 x 10^23 atoms or molecules.

Number of moles =mass in g (of an element or compound) / Mr (of the element/compound)

e.g.

How many moles are there in 66g of carbon dioxide (CO2)?

1. Mr of CO2 = 12 + (16 x 2) = 44

2. No. of moles = Mass (g) / Mr = 66 / 44 = 1.5 mol

Conservation of Mass

In a chemicas reaction mass is always conserved. No atoms are destroyed and no atoms are created. This means are the same number and types of atoms on each side of a reaction equation. No mass is lost or gained - mass is conserved.

e.g

2Li + F2 = 2LiF

In this reaction, there's 2 Lithium atoms and 2 fluorine atoms on each side of the equation. By adding up the relative formula masses of the substances on each side of the equation, you can see that mass is conserved. The total Mr of all the reactants equals the total Mr of the products.

e.g

Show that mass is conserved in this reaction: 2Li + F2 = 2LiF

1. Add up the Mr on the left side of the equation:

2 x Mr(Li) + 2 x Mr(F) = (2 x 7) + (2 x 19) = 14 + 38 = 52

2. Add up the Mr on the right side of the equation:

2 x Mr(LiF) = 2 x (7 + 19) = 2 x 26 = 52

The total Mr on the left is equal to the total Mr on the right, so mass is conserved.

If there's a change of mass, theres usually…