IMO Practice Test — Hydrogen
12 Questions • 15 min • Olympiad level
15:00
Question 1 of 12
Which statement about the isotopes of hydrogen is INCORRECT?
They have the same number of protons
They have different numbers of neutrons
They have different chemical formulae for their oxides
Tritium is radioactive
Explanation: All isotopes form the same kinds of compounds; only physical properties and reaction rates differ, not the chemical formulae.
Question 2 of 12
In which medium is the reaction 2MnO4− + 3H2O2 → 2MnO2 + 3O2 + 2OH− + 2H2O carried out, and what is H2O2's role?
acidic; oxidiser
basic; reducer
acidic; reducer
basic; oxidiser
Explanation: The OH− product shows a basic medium; H2O2 reduces Mn(+7) to MnO2, so it is the reducing agent.
Question 3 of 12
20 volume hydrogen peroxide means 1 mL of the solution liberates at STP:
20 mL of H2
20 mL of O2
20 g of O2
20 mol of O2
Explanation: Volume strength is the volume of O2 at STP given by 1 volume of the solution on decomposition; so 1 mL gives 20 mL O2.
Question 4 of 12
Which set is correctly matched (hydride : type)?
CaH2 : covalent
PdH0.6 : ionic
B2H6 : covalent
NaH : metallic
Explanation: B2H6 is a covalent (electron-deficient) hydride; CaH2/NaH are ionic and PdH0.6 is metallic.
Question 5 of 12
The water-gas shift reaction CO + H2O → CO2 + H2 is used to:
make more CO
increase the yield of H2 and remove CO
form methane
absorb hydrogen
Explanation: It converts CO to CO2 (later scrubbed out) while generating extra H2, raising purity and yield.
Question 6 of 12
The relatively high boiling point of water compared with H2S is mainly due to:
greater molar mass
stronger hydrogen bonding in water
ionic bonding
metallic bonding
Explanation: Oxygen's high electronegativity gives strong O–H···O hydrogen bonds, raising water's boiling point above H2S.
Question 7 of 12
Demineralised water from a two-bed ion-exchange plant is obtained because:
only cations are removed
only anions are removed
cations are replaced by H+ and anions by OH−, which combine to water
salts are precipitated by lime
Explanation: The cation resin gives H+ and the anion resin gives OH−; these combine to water, removing all ions.
Question 8 of 12
Why does deuterium undergo reactions more slowly than protium?
it has fewer electrons
the heavier D forms stronger bonds (kinetic isotope effect)
it is radioactive
it has a different charge
Explanation: The greater mass of D strengthens its bonds and raises activation energy, slowing reaction rates.
Question 9 of 12
The approximate dihedral angle of H2O2 changes from the gas to the solid phase as:
111° (gas) to about 90° (solid)
90° (gas) to 111° (solid)
constant at 104.5°
constant at 120°
Explanation: In the gas phase the dihedral angle is about 111°; in the solid it decreases to about 90° due to hydrogen bonding.
Question 10 of 12
The 'hydride gap' refers to the fact that:
all metals form hydrides
elements of groups 7 to 9 of the d-block generally do not form hydrides
non-metals never form hydrides
hydrogen has no hydride
Explanation: The transition metals of groups 7–9 generally fail to form stable hydrides, a region called the hydride gap.
Question 11 of 12
Which best explains why a hydrogen fuel cell is more efficient than burning hydrogen in an engine?
it produces more water
chemical energy is converted directly to electricity without a heat-engine step
it uses no oxygen
it stores hydrogen as a solid
Explanation: A fuel cell avoids the thermodynamic (Carnot) losses of a heat engine by converting chemical energy straight to electrical energy.
Question 12 of 12
When 1 mol of CaH2 reacts completely with water, the number of moles of H2 produced is:
1
2
3
4
Explanation: CaH2 + 2H2O → Ca(OH)2 + 2H2; the H− and H+ combine, giving 2 mol H2.