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Question
A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapor density is 29, find its molecular formula. [C = 12, H = 11]
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Solution
| Element | Percentage | Molecules | Simple ratio | Ration | Simple whole ratio |
| Carbon | 82.76 | `82.76/12` = 6.9 | `6.89/6.89` = 1 | `6.9/6.9` = 1 | 2 |
| Hydrogen | 17.24 | `17.24/1` = 17.24 | `17.24/6.89` = 2.5 | `17.24/6.9` = 2.5 | 5 |
Empirical formula = C2H5
Empirical formula mass = (12 × 2) + (1 × 5)
= 24 + 5
= 29
Vapour density = 29 ...(Given)
Molecular weight = 2 × Vapour density
Molecular weight = 2 × 29
= 58 gm
Molecular formula mass = n × Empirical formula mass
⇒ n = `"Molecular formula mass"/"Empirical formula mass"`
= `58/29`
= 2
Molecular formula = n × Empirical formula
= 2 × C2H5
= C4H10
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