Noadswood Science, 2012

Mass, Moles & Formula

To know how to calculate formula mass and work out the percentage of an element within a compound

Wednesday, April 19, 2023

Mass Number

Each atom consists of a nucleus containing protons and neutrons, with electrons arranged around it.

Protons and neutrons have a relative mass unit of 1

Electrons have a very small mass compared to protons and neutrons with a mass unit of 1/2000th

Most of the mass of an atom is found in its nucleus: - The mass number of an atom is the total number of

protons and neutrons it contains The atomic number (also called the proton number)

is the number of protons it contains

Chemical Symbol

The full chemical symbol for an element shows its mass number at the top, and atomic number at the bottom, e.g. carbon

It tells us that a carbon atom has six protons (it will also have six electrons, because the number of protons and electrons in an atom is the same)

The symbol also tells us that the total number of protons and neutrons in a carbon atom is 12

*You can work out the number of neutrons from the mass number and atomic number – in this example, it is 12 - 6 = 6 neutrons

Relative Atomic Mass (Ar)

The atoms of each element have a different mass – the relative atomic mass is a way of saying how heavy atoms are in comparison to others

Carbon is given a relative atomic mass (RAM) of 12

The RAM of other atoms compares them with carbon

Element Symbolx Heavier Than Carbon

(C)RAM

Helium He 1/3 4

Beryllium Be 3/4 9

Molybdenum Mo 8 96

Krypton Kr 7 84

Oxygen O 1 & 1/3 16

Silver Ag 9 108

Calcium Ca 3 & 1/3 40

Formula

Formulas tell you how many atoms are used but not the mass of each of the elements

For example in the reaction: -

Mg + 2HCl → MgCl2 +H2O

This shows us we need twice as many hydrogen and chlorine atoms, but not the masses needed (because different atoms have different masses)

Even though we are using more hydrogen atoms the mass of 2 atoms will be less than the mass of 1 magnesium

Relative Formula Mass (Mr) Relative formula mass is the sum of all the relative

atomic masses added together

For example, MgCl2 has a relative formula mass of 95: -

MgCl2

*Relative atomic mass for any element can be found using the periodic table – relative formula mass is also ‘molar mass’…

Mg = 24 Cl = 35.5 x 2

Relative Formula Mass

Work out the following relative formula masses using the atomic masses given (N = 14; H = 1; Na = 23; O = 16; Mg = 24; Ca = 40): - Ammonia NH3

Sodium oxide Na2O

Magnesium hydroxide Mg(OH)2

Calcium nitrate Ca(NO3)2

Ammonia = 14 + (3 x 1) = 17 Sodium oxide = (2 x 23) + 16 = 62 Magnesium hydroxide = 24 + 2 x (16 + 1) = 58 Calcium nitrate = 40 + 2x (14 + (3 x 16)) = 164

% Mass

Percentage composition is just a way to describe what proportions of the different elements there are in a compound

If you have the formula of a compound, you are able to work out the percentage by mass of an element in it using relative atomic mass and relative formula mass…

% mass of an element in a compound = Ar x number of atoms (of element) x 100

Mr (of whole compound)

% Mass Example – CO2

% of oxygen in carbon dioxide First find the atomic masses – C = 12 and O = 16 Formula = CO2 Number oxygen atoms = 2 Atomic mass of O = 16 Formula mass CO2 = 12 + (16 x 2) = 44

% oxygen is therefore…

16 x 2 x 100

44

= 72.7%

% Mass Example – Na2CO3

% of sodium in sodium carbonate First find the atomic masses – Na = 23, C = 12 and O =

16 Formula = Na2CO3 Number sodium atoms = 2 Atomic mass of Na = 23 Formula mass Na2CO3 = (23 x 2) + 12 + (16 x 3) = 106

% sodium is therefore…

23 x 2 x 100

106

= 43.4%

Moles

The mole is a name given to a Avogadro's number: 6.023 x 1023 or 602,300,000,000,000,000,000,000

When you have precisely this number of atoms or molecules (of any element or compound) they weigh exactly the same number of grams as the relative atomic mass of the element or compound

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

Moles

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

For example: - Carbon has an Ar of 12 – 1 mole of carbon weighs 12g

Iron has an Ar of 56 – 1 mole of carbon weighs 56g

Carbon dioxide has a Mr of 44 – 1 mole of carbon dioxide weighs 44g

This means that 12g of carbon, 56g of iron, 44g of carbon dioxide etc… all contain the same number of atoms – 1 mole!

Moles Within A Mass

The number of moles within a given mass is worked out using the following equation: -

Number of moles = Mass (g) of element or compound

Mr of element or compound

For example, to calculate the number of moles in 42g of carbon…

Number of moles = 42g 12

= 3.5 moles

Molar Solutions

A 1 molar solution contains 1 mole per litre

Number of moles = volume (litres) x molarity solution

For example – how many moles are in 185cm3 of a 2M solution?

0.185 x 2

= 0.37 moles