WAEC Chemistry 2020 Objective — Question 23

Question 23 of 50 from the West African Examinations Council (WAEC) Chemistry 2020 Objective paper, with the correct answer and a full explanation.

An alkanol containing 60% carbon by mass would have a molecular formula [H=1.0, C=12.0, O=16.0]

A. CH3OH
B. C2H5OH
C. C3H7OHCorrect
D. C4H9OH

Explanation

The general molecular formula of alkanols is CnH2n+1OH. Since the percentage of carbon is 60%: 60%=(nCarbon)/(CnH2n+1OH) x100%=12n/(12n+1(2n+1)+16+1) x100%. Solving 60/100=12n/(14n+18) gives 0.6(14n+18)=12n, 8.4n+10.8=12n, 3.6n=10.8, n=3. The formula of the alkanol is C3H7OH.

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