IB DP Chemistry Structure 1.4 Counting particles by mass: The mole IB Style Question Bank SL Paper 2

(i)     Empirical formula:

simplest (whole number) ratio of atoms/moles of each element present in a compound/molecule;

Molecular formula:

actual numbers of atoms/moles of each element present in a compound/molecule / whole number multiple of empirical formula;

(ii)     \(n{\text{(C)}} = 4.54{\text{ (mol), }}n{\text{(H)}} = 9.11{\text{ (mol)}}\) and \(n{\text{(O)}} = 2.27{\text{ (mol)}}\);

\({{\text{C}}_{\text{2}}}{{\text{H}}_{\text{4}}}{\text{O}}\);

Accept other valid method for calculation.

(iii)     \({{\text{C}}_{\text{4}}}{{\text{H}}_{\text{8}}}{{\text{O}}_{\text{2}}}\);

(iv)     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{C}}{{\text{H}}_{\text{2}}}{\text{COOH}}\);

Accept full or condensed structural formulas.

(v)     \({\text{C}}{{\text{H}}_{\text{3}}}{\text{C}}{{\text{H}}_2}{\text{COOC}}{{\text{H}}_3}/{\text{C}}{{\text{H}}_3}{\text{COOC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}/{\text{HCOOC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}/{\text{HCOOCH(C}}{{\text{H}}_3}{{\text{)}}_2}\);

Accept full or condensed structural formulas.

(vi)     Stronger bond:

C=O/double bond;

Longer bond:

C–O/single bond;

(i)     methoxyethane/ \({\text{C}}{{\text{H}}_3}{\text{OC}}{{\text{H}}_2}{\text{C}}{{\text{H}}_3}\) as there are only dipole-dipole forces (and van der Waals’ forces) between molecules;

propan-1-ol has hydrogen bonding between molecules;

hydrogen bonding is stronger than dipole-dipole forces;

(ii)     propan-1-ol/ \({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{C}}{{\text{H}}_2}{\text{OH}}\) as it has a smaller hydrocarbon chain;

the longer (non-polar) carbon chain in hexan-1-ol decreases the attraction between the alcohol and the (polar) water molecules / OWTTE;

graphite:

forms flat hexagonal rings / layers of carbon atoms each (covalently) bonded to 3 other carbon atoms / trigonal planar around C / C has \({\text{s}}{{\text{p}}^{\text{2}}}\) hybridization;

layers are held together by weak intermolecular/van der Waals’ forces;

layers can slide over each other;

delocalization of electrons / free moving electrons;

diamond:

all carbon atoms are (covalently) bonded to 4 other carbon atoms / tetrahedral around C / C has \({\text{s}}{{\text{p}}^{\text{3}}}\) hybridization;

strong covalent bonds;

no delocalized electrons / OWTTE;

There were some vague and convoluted definitions in (a)(i) but thereafter the calculations were well done. Where difficulty was found, was in the formula of an ester in (v), (AS 10.1.11).

The answers to (b)(i) were reasonable, although it was common to state that the intermolecular bonding in methoxyethane is van der Waals‘. Some G2s took issue with the examination of ethers in organic chemistry; it was, in fact, examined under AS 4.3.2. In (ii), some mentioned a “larger molecule” rather than a “longer chain” and few were able to explain the attraction (or lack thereof) between the organic molecule and water.

Part (c) suggested that there is work to be done on understanding the structures of graphite and diamond. One particular mark lost was not to state that the reason diamond is hard is because the covalent bonds are strong.