1.2 The Mole Concept
Syllabus
What does this mean?
What is a mole?
The mole is the SI unit of substance.
6.02 x1023 (or 602,000,000,000,000,000,000,000) atoms is a mole of atoms .
6.02 x1023 molecules is a mole of molecules.
6.02 x1023 ions makes a mole of ions etc .
This number is called AVOGADRO’S NUMBER (NA or L).
Video
QUESTION
1. a. How many molecules in a mole of water?
b. How many atoms in a mole of water?
2. a. How many molecules in a mole of H2SO4?
b. How many atoms in a mole of H2SO4?
Answers
1.a 6.02 x1023 molecules (NA)
b. Each water molecule has three atoms. So,
3 x NA = 3 x 6.02 x1023 = 1.81 x1024 atoms
2.a 6.02 x1023 molecules (NA)
b. Each molecule has 7atoms. So,
7 x NA = 3 x 6.02 x1023 = 4.21 x1024 atoms
Relative Formula Mass(Mr)
You’ll almost certainly have studied this too.
Example.
QUESTIONS
Find the Mr of:
i) (NH4)3PO4 ii) Ca(HCO3)2 iii) Al(HSO4)2
iv) CuSO4.5H2O v) Na2B4O7.10H2O vi) Mg3(PO4)2.22H2O
Answers
i) (NH4)3PO4 = ((14+4) x 3) + 31 + (4x16) = 149
ii) Ca(HCO3)2 = 40 + ((1+12+48)x2) = 162
iii) Al(HSO4)2 = 27 + ((1+ 32+64) x2) = 221
iv) CuSO4.5H2O =
v) Na2B4O7.10H2O = (23x2) + (11x4) + (16x7) + (10 x18) = 382
vi) Mg3(PO4)2.22H2O = (24 x 3) + ((31 + 64)x2) + (22 x 18) = 658
Basic Mole Calculations
If you don’t already know...
LEARN :
MOLES = MASS/Mr
Video
Empirical Formula and Molecular Formula VIDEO
Molecules v Lattices
Only molecules have molecular formulas, obviously.
Ionic Compounds don’t form molecules, they form lattices.
Every lattice of Sodium Chloride, for instance, is a different size but what they have in common is the ratio between the Na+ and Cl- ions.
This is 1:1 so the empirical formula of Sodium Chloride is Na1Cl1 or NaCl.
There's no point trying to count all the ions in a grain of salt!
Pure metals, such as Copper, only contain one type of atom.
So the empirical formula of Copper is just Cu.
There's no point trying to count all the atoms in a lump of Copper!
The same is true of the few covalent lattice structures.
The empirical formula of both diamond and graphite is just C – because there are no other atoms involved.
There's no point trying to count all the atoms in a diamond or a pencil 'lead'!
Water does form molecules.
The molecular formula counts every atom in a molecule – H2O
The simplest ratio is also H2O so this is also the empirical formula
But Hydrogen Peroxide has a molecular formula of H2O2 but an empirical formula of HO (the simplest ratio)
Calculating Empirical Formula from Percentage Mass
Simply follow the pattern below:
Q. A compound is 40% Carbon and 53.3% Oxygen by mass. The rest is Hydrogen.
i) Find its empirical formula.
ii) If the Mr – 90 g/mol find the molecular formula
Work out the remaining %
Treat every % as if it is a mass in grams
Divide each “mass” by the Ar of the element to find moles
Divide each mole by the lowest number
Use the simplest whole number ratio as the numbers for the empirical formula
i) Answer C1H2O1 or CH2O
ii)
Find the Mr of the empirical formula
Divide the Mr of the molecular formula by the Mr of the Empirical Formula
Multiply the empirical formula by the result
Mr(CH2O) = 12 + 2 + 16 = 30
90/30 = 3
(CH2O) x 3 = C3H6O3
Example 2
Find the molecular formula of a substance which is 38.67% Carbon, 45.11% Nitrogen with rest Hydrogen. The Mr = 62 g/mol
Carbon Nitrogen Hydrogen
%/mass 38.67g 45.11g 16.22g
Moles 38.67/12 45.11/14 16.22/1
=3.22 =3.22 =16.22
Divide by 3.22/3.22 3.22/3.22 16.22/3.22
Smallest =1 =1 =5.03 = 5
EMPIRICAL FORMULA = CNH5
Mr(CNH5) = 31 g/mol
62/31 = 2 MOLECULAR FORMULA = 2 x EMPIRICAL FORMULA
MOLECULAR FORMULA = C2N2H10
QUESTION
1. A compound is found to contain 50.05 % Sulphur and 49.95 % Oxygen by weight. What is the empirical formula for this compound? The molecular weight for this compound is 64.07 g/mol. What is its molecular formula?
2. A compound is found to contain 64.80 % Carbon, 13.62 % Hydrogen, and 21.58 % Oxygen by weight. What is the empirical formula for this compound? The molecular weight for this compound is 74.14 g/mol. What is its molecular formula?
3. A compound is found to contain 31.42 % Sulphur, 31.35 % Oxygen, and 37.23 % Fluorine by weight. What is the empirical formula for this compound? The molecular weight for this compound is 102.2 g/mol. What is its molecular formula?
4. Ammonia reacts with Phosphoric Acid to form a compound that contains 28.2% Nitrogen, 20.8% Phosphorous, 8.1% Hydrogen and 42.9% Oxygen. Calculate the empirical formula of this compound.