Chemistry Calculations

• Relative atomic mass is the mass number of an element - e.g mass number of helium is 4.
• Relative formula mass is the relative atomic masses of all the elements in a compound added together - e.g CaCO3 = 40 + 12 + (16 x 3) = 100.

So, Relative formula mass for CaCO3 is 100.

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• Created by: holli
• Created on: 18-05-10 15:14

Calculating % Mass of an Element in a Compound.

Percentage mass of an element in a compound =

(RAM x No. of Atoms (of that element) / Relative Formula Mass) x 100

E.g Find the percentage mass of sodium in sodium carbonate, Na2CO3.

% Mass = (23 x 2) / 106 x 100 = 43.4%.

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Finding the Empirical Formula from Masses or Perce

1. List all elements in compound.

2. Write experimental masses/percentages.

3. Divide mass/percentage by the RAM for that particular element.

4. Turn numbers into a simple ratio by multiplying or dividing by well chosen numbers.

5. Get ratio in simplest form.

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Emperical Formula Example.

E.g Find the empirical formula of iron oxide produced when 44.8g of iron reacts with 19.2g of oxygen.

1. Fe O

2. 44.8 19.2

3. 44.8/56=0.8 19.2/16=1.2

4. Times ten, Then Divde 4.

8 12

2 3.

5. Therefore, Fe2O3.

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