Mr and the percentage of carbon in a hydrocarbon or alcohol, moles and masses of carbon dioxide from burning a fuel, gas volumes, and estimating boiling points from a homologous series.
What you will learn
- Calculate the Mr and percentage by mass of carbon of an organic compound
- Calculate the mass of carbon dioxide made when a fuel burns (Higher)
- Estimate a boiling point from the trend in a homologous series
Mr and percentage of carbon
Add up the relative atomic masses of every atom in the formula.
Then:
percentage of carbon = (12 × number of C atoms) ÷ Mr × 100
You must recall this: there is no chemistry equation sheet.
- Hexane, C₆H₁₄ (C = 12, H = 1)
- Mr = 6 × 12 + 14 × 1 = 86
- mass of carbon = 6 × 12 = 72
- % carbon = 72 ÷ 86 × 100 = 83.7 % (1 d.p.)
Multiply 12 by the number of carbon atoms. 12 ÷ 86 × 100 gives 14.0 %, which is far too low for a hydrocarbon.
Along the alkanes the percentage of carbon rises (methane 75.0%, hexane 83.7%), creeping towards 85.7%: each extra CH₂ adds more carbon than hydrogen by mass, and the 2 extra hydrogens at the ends matter less in a longer chain.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Mr and percentage of carbon
- Try it: Check an Mr
- Carbon dioxide from burning a fuel (Higher)
- Estimating from a homologous series
10 practice questions, marked as you go; about 25 minutes in all.
In this unit: Organic chemistry
Crude oil and the alkanes, combustion and cracking, then alkenes, alcohols, carboxylic acids, esters and polymers for separate Chemistry, with the formula and mass calculations that go with them.