chemical equation

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Topics: Ammonia, Nitrogen, Atom, Mole
CHEMISTRY TOPIC 11

CHEMICAL CALCULATIONS

CHEMICAL CALCULATIONS

INTRODUCTION
The first part of this ‘Chemical Calculations’ topic will help us to work out QUANTITIES involved in a reaction;

For example, a manufacturer might want to know,

How much ammonia will I produce from 20 tonnes of nitrogen in the Haber Process?

To do these calculations you will need to be familiar with the term Ar (relative atomic mass), Mr, Molar mass and Mole.

Relative Atomic Mass – Ar

• The Ar tells us the average mass of an individual atom (average because of isotopes).
• The Ar for an atom has no units as it is relative
• You will always be given the Ar for an atom in any example set.

Relative formula/molecular mass – Mr

Relative formula/molecular mass – Mr

Most substances exist as;
- compounds if ionic
- molecules if covalent
We can use Ar’s to calculate the ‘average mass of ONE unit of a substance’ this is called the Mr (see also topic 9)
E.g. Calculate the Mr for sodium carbonate 3Na2CO3.

Ar (Na) = 23
Ar (C) = 12
Ar (O) = 16

The number 3 in front of the formula has been ignored because by definition Mr is the mass of ONE unit of a substance whether the substance is ionic or covalent.
Molar mass

This is the mass of one mole of a substance (see later). Molar mass is simply the Ar or Mr expressed in grammes. The molar mass for the above example is 106g.
The Mole

When considering quantities in Chemistry it is unrealistic to talk in terms of units such as atoms, molecules, repeats of ionic formulae.
A chemist called Avodagro chose a quantity of units (0.6 million, million, million, million/6.023 X 1023 based on the number of atoms in 12g carbon) and called it the mole. To put this into perspective, one mole of marbles would form a layer 1500km deep over the U.K. and Ireland. A mole is a quantity that you could see.
Lesson 1
Content Using Ar, Mr to find Molar mass in order to

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