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AQA GCSE · Question 09.6 · Organic chemistry
Figure 9 shows part of the displayed formula for ethanol. Complete Figure 9.
Figure 9 shows part of the displayed formula for ethanol. Complete Figure 9.
How to approach this question
1. The formula is for ethanol, C₂H₅OH.
2. The partial structure shows H₃C-C-.
3. The second carbon atom needs to form a total of four bonds. One is already used to bond to the first carbon.
4. The "ol" ending in ethanol signifies an alcohol, which has an -OH functional group. Add an -OH group to the second carbon.
5. The second carbon now has two bonds (C-C and C-O). It needs two more bonds to make four. Add two hydrogen atoms to fill the remaining bonds.
Full Answer
The completed structure has an -OH group and two H atoms attached to the right-hand carbon.
Ethanol has the chemical formula C₂H₅OH. The name tells us it has two carbon atoms ("eth-") and is an alcohol ("-ol"), meaning it contains the hydroxyl (-OH) functional group.
The displayed formula must show all atoms and all covalent bonds.
- The first carbon atom is correctly shown with three hydrogen atoms attached (H₃C-).
- This is attached to the second carbon atom (-C-).
- This second carbon must have the -OH group attached.
- Carbon atoms always form four covalent bonds. The second carbon is bonded to the first carbon and the oxygen atom, so it needs two more bonds. These are filled by two hydrogen atoms.
The structure of the group attached to the first carbon is therefore -CH₂OH.
Common mistakes
✗ Forgetting the -OH group.
✗ Drawing the wrong number of bonds to the carbon atom (it must be 4).
✗ Attaching the oxygen and hydrogen incorrectly (e.g., C-H-O instead of C-O-H).
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