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Question
A gaseous compound of nitrogen and hydrogen contains 12.5% hydrogen by mass. Find the molecular formula of the compound if its relative molecular mass is 37.
[N = 14, H = 1].
Solution
Calculation of empirical formula:
Percentage of hydrogen = 12.5%
Percentage of nitrogen = 100 − 12.5 = 87.5%
| Element | Atomic weight | Percentage | Relative Number of moles | Simplest whole no ratio |
| N | 14 | 87.5 | `87.5/14` = 6.25 | `6.25/6.25` = 1 |
| H | 1 | 12.5 | `12.5/1` =12.5 | `12.5/6.25` = 2 |
Empirical formula = NH2
Given molecular mass = 37
Empirical formula mass = 14 + 2 = 16
n = `"Molecular mass"/"Empirical mass"`
= `37/16`
∴ n = 2.31
Hence, Molecular Formula = (NH2)2.31
= N2.31H4.62
Notes
This formula goes against the avogadro's law and hence cannot exist.
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