Short answerEasy2 marks
The percentage composition of a gas is: Nitrogen 82.35%, Hydrogen 17.64%. Find the empirical formula of the gas. [N = 14, H = 1]
Topic: Mole concept, gas volumes and stoichiometry
Previous-year question practice database
Answer
Exam answer
Given: N = 82.35 %, H = 17.64 %. [N = 14, H = 1]
| Nitrogen | Hydrogen | |
|---|---|---|
| Percentage | 82.35 | 17.64 |
| ÷ atomic mass | 82.35 ÷ 14 = 5.88 | 17.64 ÷ 1 = 17.64 |
| ÷ smallest (5.88) | 5.88 ÷ 5.88 = 1 | 17.64 ÷ 5.88 = 3 |
Simplest whole-number ratio N : H = 1 : 3
Empirical formula = NH₃
Explanation
The empirical-formula method is always the same four rows: take the percentages, divide each by its atomic mass to get moles, divide every result by the SMALLEST of them, and round to whole numbers. Set it out as a table and the marks follow the rows.
The step students skip is the third one — dividing by the smallest. Stopping at 5.88 and 17.64 leaves you with no formula. And divide by the smallest, not by the first one in the table; here they happen to be the same, but often they are not.
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