IMO Practice Test — Carbon and its Compounds
12 Questions • 15 min • Olympiad level
15:00
Question 1 of 12
The number of structural isomers of pentane C5H12 is:
2
3
4
5
Explanation: Pentane has three isomers: n-pentane, isopentane (2-methylbutane) and neopentane (2,2-dimethylpropane).
Question 2 of 12
A hydrocarbon has the molecular formula C4H8. It is most likely a(n):
alkane
alkene or cyclic alkane
alkyne
alcohol
Explanation: C4H8 fits CnH2n, which is the formula for an alkene (one double bond) or a cyclic alkane.
Question 3 of 12
Which has the highest percentage of carbon by mass?
CH4
C2H6
C2H4
C2H2
Explanation: In C2H2, carbon is 24/26 = 92.3%, the highest; this is why ethyne burns with a very sooty flame.
Question 4 of 12
Decolourisation of bromine water is a test for:
saturated hydrocarbons
unsaturated hydrocarbons
alcohols
carboxylic acids
Explanation: Unsaturated hydrocarbons add bromine across the double/triple bond, decolourising bromine water.
Question 5 of 12
A 14 u mass difference between two compounds of the same family indicates they are:
isomers
consecutive homologues
allotropes
the same compound
Explanation: A difference of one CH2 (14 u) means they are consecutive members of a homologous series.
Question 6 of 12
Heating ethanol with excess concentrated H2SO4 at 170 °C gives mainly:
ethanoic acid
ethene
ethane
ethyl ethanoate
Explanation: Excess hot conc. H2SO4 dehydrates ethanol: C2H5OH -> C2H4 + H2O (ethene).
Question 7 of 12
Which reagent can distinguish ethanol from ethanoic acid?
sodium metal
sodium hydrogen carbonate
blue litmus
water
Explanation: Ethanoic acid releases CO2 (brisk effervescence) with NaHCO3, while ethanol does not react; sodium reacts with both.
Question 8 of 12
The number of covalent bonds in a molecule of ethane C2H6 is:
6
7
8
5
Explanation: Ethane has six C-H bonds plus one C-C bond, giving seven covalent bonds in all.
Question 9 of 12
Saponification is the reaction of an ester with a base to give:
an acid and water
an alcohol and the salt of a fatty acid
an aldehyde and water
two esters
Explanation: Saponification (alkaline hydrolysis of an ester) gives back the alcohol and the sodium salt of the acid (soap).
Question 10 of 12
Why is diamond used in cutting tools but graphite is used as a lubricant?
both are soft
diamond is a rigid 3-D network (hard) while graphite has sliding layers (soft)
graphite is harder than diamond
diamond conducts heat poorly
Explanation: Diamond's rigid tetrahedral network makes it extremely hard; graphite's layers slip over each other, making it a lubricant.
Question 11 of 12
Detergents clean better than soaps in hard water because the calcium and magnesium salts of detergents are:
insoluble
soluble in water
gaseous
acidic
Explanation: Unlike soaps, detergents form soluble Ca/Mg salts, so they do not produce scum in hard water.
Question 12 of 12
An organic compound X gives brisk effervescence with NaHCO3 and turns blue litmus red. X most likely contains the group:
−OH
−CHO
−COOH
−CO−
Explanation: Acidic behaviour (red litmus) and CO2 release with NaHCO3 indicate the carboxylic acid group -COOH.