C4

The relative atomic mass (Ar) is the average mass of an element. 

The relative formula mass (Mr) is the overall mass of all the elements in a substance.

To find the relative formula mass you follow these steps:

1) Find the formula for the compund.

Sulphuric Acid = H₂SO₄

2) Find the relative atomic masses for each of the elements.

A_r of H = 1

A_r of S = 32

A_r of O = 16

3) Add all the elements together, multiplying them if needed.

(1x2) + 32 + (16x4) = 2 + 32 + 64 = 98

A mole is simply the relative atomic mass of relative formula mass expressed in grams.

A mole of any substance always carries the same amount of atoms, molecules or ions. It's a big number (called the Avogadro Number). In standard form it's written as 6.02 x 10²³ (that's 602 hexillion!). 

To find the number of moles in a substance, you use this equasion:

number of moles = mass (g) divided A_r

or

number of moles = mass (g) divided M_r

Sometimes, you will need to find the mass of a substance when you are given the number of moles. You would use this equasion:

mass (g) = number of moles x A_r or number of moles x M_r

When you want to know how much of each substance is in a chemical equation, we use balanced equations.

For example, when hydrogen molecules (H2) react with chlorine molecules (CI2), we get hydrogen chloride molecules (HCI).

H2 + CI2 = HCI

This equation shows the reactants and the product, but if you look, there are 2 hydrogen atoms (H2) and 2 chlorine atoms (CI2) before the chemical reaction takes place, but after there are only 1 of each.

So, we fix it by adding a 2 before HCI to show there are 2 of each of the elementts:

H2 + CI2 = 2HCI

The mass of end product that a chemical reaction produces is called its yield.

The theoretical yield is the maximum mass of product that can be made from a chemical reaction. We calculate this by adding up all the relative formula masses in an equation.

However, chemical reactions don't always go right, or material can be lost during the experiment. We often think about reactions in terms of percentage yield. This compares the actual final mass of product compared to the theoretical yield. The formula for this is:

percentage yield = actual mass of product divided by theoretical yield x 100%

There are several things that could affect the theoretical yield such as:

 - some product may be lost in handling

 - some reactants may give unwanted products

 - some of the product may be lost while being seperated from the reaction mixture

Chemical companies want to use reactions with high percentage yields, wasting as little energy and raw meterials as possible. 

The atom economy is a measure of how much of the atoms in the starting materials end up in the end product.

The formula to work out the atom econony is:

percentage atom econony = relative formula mass of desired proudct from equasion divided by sum of relative formula masses from reactants x 100

Concentration is measured in g/dm³.

The equasion for calculating concentration is:

concentration (g/dm³) = amount of solute (g) divided by volume of solution (dm³)

If you are working in cm², then divide it by 1000 to convert to dm³.