# Chemistry C2.3

- Created by: juddr12
- Created on: 16-04-14 16:28

**C2 3.1**

The total of protons and neutrons are the **mass number**. This is because the mass of electrons are very close to 0, so we consider them to be 0. The atomic number represents the amount of protons and electrons it has. The atomic number is the number that defines what an element is. However, if the amount of neutrons changes, the element becomes an **isotope**.

**C2 3.2**

Relative atomic mass or Ar is the number shown on the periodic table. One mole of an element = its Ar.

The **relative formula mass or Mr** is when you add the atomic masses of atoms.

Using **moles** are useful because it tells us how much of a substance reacts and how much product we get.

**C2 3.3**

To get a percentage of a element/s from a compound just simply work out the Ar (or Mr if multiple elements are wanted) of the element you want and then simply divide it by the Mr of the entire compound. Times the answer by 100 to get a **percentage**.

For example, if you were to work out the percentage of carbon in carbon dioxide. You get the Ar of one carbon = 12. You then get the Mr of the entire compound: CO2 = 12 + 16 + 16= 44.

12/44 = 0.273 x 100

= **27.3%**

**HIGHER:**

However, to work out the **empiracal formula** (a simple ratio of elements in a compound) you basically do the same thing but put them side by side.

You find out the percentage out of **100g** of the compound. You then divide each element by its Ar (e.g. 80 grams of carbon/12). You then divide all the elements by the smallest amount of element. So one of elements equal 1 while the others equal higher. If you get any decimal places, try multiplying throughout by a small number to get them to whole numbers, but make sure that they are the lowest number that they can be at. If the decimals are also quite close to a whole number, just round it **up or down**. You then write out the numbers you get into an emperical formula, like so: "CH3" (1 for Carbon, 3 for Hydrogen).

For example, to…

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