A hydrocarbon compound contains 92.3% carbon Determine its empirical formula [H = 1.00; C = 12.0]

Step 1: Analyze the question A hydrocarbon contains only carbon and hydrogen. If carbon is 92.3%, then hydrogen is 100 − 92.3 = 7.7%.

Step 2: Concept To find the empirical formula:

1. Assume 100 g of the compound, so the percentages become masses.
2. Divide each mass by its atomic mass to get moles.
3. Divide by the smallest number of moles to get the simplest ratio.

Step 3: Solution

| Element | Mass (g) | Moles | Ratio | |---|---|---|---| | C | 92.3 | 92.3 ÷ 12.0 = 7.69 | 7.69 ÷ 7.69 = 1 | | H | 7.7 | 7.7 ÷ 1.00 = 7.7 | 7.7 ÷ 7.69 ≈ 1 |

The ratio of C : H is 1 : 1, so the empirical formula is CH.

Step 4: Answer The correct option is A. CH. For example, benzene C₆H₆ and ethyne C₂H₂ both have this empirical formula.)

Tip: When a hydrocarbon is about 92% carbon, think CH, because the mass ratio of C to H of about 12 : 1 means equal numbers of atoms. If you see about 85.7% carbon, think CH₂, and about 80% carbon, think CH₃. Memorizing these saves time in the exam.