Chem 2 - Bonding and Calculations -Two Formula Mass Calculations

Revision cards for a couple of Formula Mass Calculations, hope this helps! :]

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Two Formula Mass Calculations

Calculate the % mass of an element:

This just means to calculate what percentage of a compound a certain element takes up. You can use this calculation:

- % Mass of an element = (Ar x No. of atoms (of that element)) DIVIDED BY (Mr of whole compound) AND THEN times the result by 100.

An example of this:

- Find the % mass of sodium in Sodium Carbonate (Na2CO3)

Ar of Sodium = 23. (2 atoms) Ar of Carbon = 12. Ar of Oxygen = 16 (3 atoms)

Mr = (23 x 2) + 12 + (16 x 3) = 106.

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

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Two Formula Mass Calculations 2

Empirical Formula:

This is just how you can find out the number atoms for each element in a compound. To do this, first list all the elements in the compound and how much of them there are (in grams). Then, divide each of the masses of each element, by a single mole of the element. Then, divide all the values you get after the calculations, by the smallest value you get. Here is an example:

In a substance, there is: 

- 0.60g of Carbon. - 0.15g of Hydrogen. - 0.40g of Oxygen.

Divide each ones mass by a mole of the element..

- 0.60/12 = 0.05 (Carbon). - 0.15/1 = 0.15 (Hydrogen). - 0.40/16 = 0.025 (Oxygen)

Then divide each value by the smallest value..

- 0.05/0.025 = 2 (so there are 2 atoms of carbon)

- 0.15/0.025 = 6 (so there are 6 atom of carbon)

- 0.025/0.025 = 1 (so there is 1 atom of carbon)

So the formula of the substance would be C2H6O

If you are given percentages of each substance, just convert it into a total of 100g of the substance. (Eg if there was 63% Magnesium and 37% Oxygen, convert to 63g Magnesium and 37g Oxygen)

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