MC test: Theories of acids & bases

Multiple choice test on 8.1 Theories of acids & bases

Use the following 'quiz' to test your knowledge and understanding of this sub-topic. You will need access to a periodic table (Section 6 of the IB data booklet).

If you get an answer wrong, read through the explanation carefully to learn from your mistakes.

Which can be used to represent a proton in aqueous solution?

I. H+(aq)

II. H3O+(aq)

III. p(aq)

Either H+(aq) or H3O+(aq) is accepted and used by the IB.  A proton can be used to represent a proton in nuclear chemistry (e.g. as a constituent of the nucleus) but is not used in solutions.

 

Which can act as a Brønsted-Lowry base?

NH3 has a non-bonding pair of electrons on the N atom which can accept H+ to form the ammonium ion, NH4+.

 

Which can act as a Brønsted-Lowry acid?

The hydrogencarbonate ion, HCO3− can donate a proton to form the carbonate ion CO32−. The only other species that contains protons is methane and the C−H bond is too strong to form CH3− and H+.

 

Which species can act both as a Brønsted-Lowry acid and as a Brønsted-Lowry base?

The hydroxide ion, OH− can lose a proton to form the oxide ion, O2− and it can accept a proton to form water. The remaining three species can all act as Brønsted-Lowry acids but cannot accept a proton.

 

Which is the conjugate base of the dihydrogenphosphate ion, H2PO4−?

A conjugate base is the species formed when a Brønsted-Lowry acid loses a proton. When H2PO4− loses a proton it forms the HPO42− ion.

 

Which is a conjugate acid -base pair?

The conjugate base is the species formed when the Brønsted-Lowry acid loses one proton.

 

Magnesium ammonium phosphate is a sparingly soluble salt that can be formed in urine. The simplified equation for its formation is:

Mg2+(aq) + NH4+(aq) + HPO42-(aq) + H2O(l) ⇄ MgNH4PO4 (s) + H3O+(aq)

Which species are acting as Brønsted-Lowry acids in this reversible reaction?

Both HPO42− and H3O+ donate a proton (PO43− and H2O are their respective conjugate bases). Mg2+ and NH4+ remain unchanged.

 

Which is amphoteric but not amphiprotic?

All the species are amphoteric as they can behave as an acid or a base and all but Al2O3 are amphiprotic as they can donate or accept a proton.

 

Ethylamine can react reversibly with water:

C2H5NH2(aq) + H2O(l) ⇄ C2H5NH3+(aq) + OH−(aq)

Which row contains the correct description for each species?

Row C2H5NH2(aq) H2O(l) C2H5NH3+(aq) OH−(aq)
1 acid conjugate base acid conjugate base
2 acid conjugate base conjugate base acid
3 conjugate base acid acid conjugate base
4 conjugate base acid conjugate base acid

 

H2O and C2H5NH3+ are each donating a proton so are functioning as Brønsted-Lowry acids. OH− and C2H5NH2 are each accepting a proton so are functioning as the conjugate bases of H2O and C2H5NH3+ respectively.

 

Hydrogen sulfide is slightly soluble in water forming both the hydrogen sulfide ion and the sulfide ion.

H2S(g) + H2O(l) ⇄ HS−(aq) + H3O+(aq)

HS−(aq) + H2O(l) ⇄ S2−(aq) + H3O+(aq)

Which species is showing amphiprotic behaviour in these reactions?

HS− is accepting a proton from H3O+ in the first equation and donating a proton to water in the second equation.

 

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