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Percentage composition and empirical formula

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Empirical formula – Higher tier

You should be able to calculate the empirical formula [empirical formula: Simplest whole number ratio of each type of atom in a compound. of a compound [compound: A substance formed by the chemical union (involving bond formation) of two or more elements. if you are given the mass [mass: The amount of matter an object contains. Mass is measured in 'kg'. of each element [element: A substance made of one type of atom only. it contains. Here is an example:

Example

3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?

(Ar of S = 32 and Ar of O = 16)

Step

Action

Result

1

Write the element symbols

S

O

2

Write the masses

3.2 g

6.4 – 3.2 = 3.2 g

3

Write the Ar values

32

16

4

Divide mass by Ar

3.2 ÷ 32 = 0.1

3.2 ÷ 16 = 0.2

5

Divide by the smallest number

0.1 ÷ 0.1 = 1

0.2 ÷ 0.1 = 2

6

Write the formula

SO2

The action at Step 5 usually gives you the simplest whole number ratio straightaway. Sometimes it does not, so you might get 1 and 1.5. In this example, you would multiply both numbers by 2, giving 2 and 3 (instead of rounding 1.5 up to 2).

You should be able to calculate the empirical formula of a compound if you are given its percentage composition by mass.

To do this, assume you have 100 g in total of the compound. That way, each percentage becomes a mass in grams, and you can then just use the method shown above.

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