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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There are some other calculations to be included, e.g. calculate chemical quantities involving empirical formulae, reacting masses and percentage yield. calculating the atom economy for industrial processes and be able to evaluate sustainable development issues related to this economy. But still this was useful. Thanks!

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