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Question
Two oxides of a metal (M) have 20.12% and 11.19% oxygen. The formula of the first oxide is MO. Determine the formula of the second oxide.
Solution
Calculation of molar mass of M from first oxide :
Let us assume the atomic mass of Mas x.
Atomic mass of Oxygen = 16
| Element | Percentage | Relative number of moles | Simplest mole ratio |
| M | 100 - 20.12 = 79.88 | 79.88/x | 79.88/1.25x |
| O | 20.12 | 20.12/16=1.25 | 1.25/1.25 = 1 |
So, molar mass of M, x = 63.5
Calculation Of formula of second oxide :
| Element | Atomic mass | Percentage | Relative number of moles |
Simplest |
Whole number ratio |
| M | 63.5 | 88.81 | 88.81/63.5 = 1.4 | 1.4/0.69 = 2 | 2 |
| O | 16 | 11.19 | 11.19/16 = 0.69 | 0.69/0.69 =1 | 1 |
So formula of second oxide is M2O.
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