S1.4 Stoichiometry

Avogadros Constant

Avogadros Constant

1 mol = 6.02 × 10²³ atoms of that element

Avogadro’s equation

n = N/N_A

_{n is number of mols}

_{N is the given number of elements (or the number of elements you have to find out)}

_{Na is the Avogadro’s constant}

Relative moldecular mass, Mr

Adding the Mass number (M) for each element (e.g H20 has 2×1.01 + 16)

It has no units, unlike molar mass, M which is gmol-1

How to find number of element in compound (e.g how many H atoms in H20)

Find the number of mols for the compund, multiply by the number of that element in the formula (e.g times by two for H20 because 2 H atoms)

Then times by Avogadro’s constant.

Calculating Empirical Formula

It is the smallest ratio of elements (eg H20 —> HO)

To calculate:
1) Let mass of compund = 100g and let the percentage of each element = the mass of each element (eg 75% = 75g of Fe)

2) Calculate n of each element and find simplest ratio (this is empirical formula)

Percentage Composition

How much of each element is in the compound

1) Let n of compund = 1 mol
2) m = M x n for each element (remember to times number of moles by 2 if there is 2 of the element eg H2)

3) mass of one element / total mass x 100% gives pecentage for one element

4)100 - percentage for the composition of the other element

Molecular Formula

How many there actually is

eg: CxHy ←x and y values will be given, or empirical formula

{ (12.01 x) + (1.01 y) }

Given mass / { } = a rough whole number, times x and y by this to get molecular formula