VCAA Chemistry How are organic compounds analysed and used?

This incorrect answer is obtained by mistakenly swapping the volumes of the acid and base in the concentration calculation.

This error occurs if the volumes of the acid and base are swapped and the 1:3 stoichiometric ratio between citric acid and NaOH is ignored.

This incorrect result comes from multiplying the moles of NaOH by 3 instead of dividing by 3 to find the moles of the triprotic citric acid.

3-methylbutanoic acid has four unique sets of protons, which would produce a singlet (OH), a doublet (CH3 groups), a multiplet/nonet (CH), and another doublet (CH2).

2-chloro-2-methylpropane has only one unique set of protons (nine equivalent methyl protons), which would produce a single peak (singlet) in its $^1\text{H}$ NMR spectrum.

1,2-dichloro-2-methylpropane has two unique sets of protons that are not adjacent to any other protons, which would result in a spectrum with two singlets.

Zwitterions are molecules with both positive and negative charges that net to zero, typically occurring at an amino acid's isoelectric point. In highly acidic conditions, amino acids become positively charged cations, not zwitterions.

Acids do not esterify enzymes into smaller molecules. Breaking down an enzyme's protein chain into smaller molecules would involve the hydrolysis of peptide bonds, not esterification.

While acidic conditions do protonate carboxylate groups into neutral carboxyl groups, this is only a partial explanation. The loss of enzyme effectiveness is due to the overall change in charges across all ionizable groups, which disrupts the active site.

This is a solution of Iron(III) nitrate. Neither $Fe^{3+}$ nor $NO_3^-$ ions react with bromine water, so the solution would not be decolourised.

Chloride ions ($Cl^-$) cannot reduce bromine ($Br_2$) because chlorine is a stronger oxidising agent than bromine. Therefore, no reaction occurs to remove the bromine colour.

Solvent extraction relies on differences in solubility, which would likely be ineffective here since both ethanol and the other compounds are polar and may dissolve in similar solvents.

Simple distillation is only effective for separating liquids with significantly different boiling points (typically a difference of at least 25°C).

Adding solvent extraction is unnecessary and less efficient, as fractional distillation alone is the standard and most effective method for separating liquids with similar boiling points.

Carbohydrates are organic compounds like sugars and fibers used primarily for energy and structure, not for catalyzing reactions.

Starches are complex carbohydrates (polysaccharides) used for energy storage in plants, whereas enzymes are the proteins that might break them down.

Lipids include fats, oils, and waxes used for long-term energy storage and membrane structure, but they do not function as biological catalysts.

Rinsing the pipette with NaOH would neutralize some of the citric acid before it is transferred to the flask. This decreases the required NaOH titre, resulting in a lower calculated concentration.

Rinsing the pipette with the solution it will transfer (citric acid) is the standard correct procedure and ensures the concentration remains accurate.

Rinsing the conical flask with NaOH adds extra base to the flask, which neutralizes some of the citric acid before titration begins. This decreases the required NaOH titre, leading to a lower calculated concentration.

IR radiation causes changes in the vibrational states of molecules, not the spin states of electrons (which is associated with Electron Paramagnetic Resonance spectroscopy).

The wavenumber of a bond's vibration depends on both the bond strength (force constant) and the reduced mass of the atoms involved, as described by Hooke's Law.

Transmittance and absorbance are inversely related; high transmittance means that most of the light passed through the sample, indicating low absorption.

Primary structure refers to the linear sequence of amino acids held together specifically by covalent peptide bonds, not by the various side-chain interactions listed.

Secondary structure, such as $\alpha$-helices and $\beta$-sheets, is primarily stabilized by hydrogen bonds between the backbone amide and carbonyl groups, rather than interactions between side chains.

Quaternary structure refers specifically to the arrangement of multiple polypeptide subunits; while these forces hold subunits together, the question describes the forces defining the 3D fold of a single chain (tertiary structure).

The pH equals the p$K_a$ at the half-equivalence point, where exactly half of the weak acid has been converted to its conjugate base.

The pH changes most rapidly at the equivalence point rather than stopping; it only levels off significantly after excess titrant is added.

The point where the indicator changes colour is called the end point, which is an experimental approximation that should ideally be close to the equivalence point.

Changing the polarity of the mobile and stationary phases alters the separation method (switching to normal phase chromatography) but does not provide the quantitative reference data needed to calculate concentration.

Retention times and peak sizes are highly dependent on the specific instrument, column age, and exact experimental conditions, so published data cannot be reliably used for quantitative analysis.

While reducing the temperature might improve the separation (resolution) of the peaks, it does not provide the reference standards required to calculate the actual concentration of the compound.

Incorrect. Denaturation can be caused by many other factors besides temperature and acids, such as bases, heavy metals, organic solvents, and mechanical agitation.

Incorrect. Denaturation is most commonly an irreversible process, such as cooking an egg, rather than a reliably reversible change.

Incorrect. Denaturation does not break the covalent peptide bonds of the primary structure; a different process called hydrolysis is required to break down a protein's primary sequence.

This incorrect volume results from using a 2:5 molar ratio of oxalic acid to permanganate, which is the inverse of the actual stoichiometric ratio.

This assumes a 1:1 stoichiometric ratio between oxalic acid and potassium permanganate, ignoring the number of electrons transferred in the redox reaction.

This is a miscalculation. The correct volume must be determined by balancing the electrons transferred in the oxidation and reduction half-reactions.

This order represents the highest to lowest retention time, which would be the correct order if the stationary phase were polar (normal-phase HPLC).

Hexane is the most non-polar compound, meaning it will interact most strongly with the non-polar stationary phase and have the highest, not lowest, retention time.

While hexane correctly has the highest retention time, hexan-1-ol is more polar than hexan-2-one due to its ability to form hydrogen bonds, so hexan-1-ol will elute before hexan-2-one.

Primary structure refers to the linear sequence of amino acids connected by covalent peptide bonds, not hydrogen bonds.

Tertiary structure is the overall 3D shape determined primarily by interactions between amino acid side chains (R-groups), such as disulfide bridges, hydrophobic interactions, and ionic bonds.

Quaternary structure involves the arrangement of multiple polypeptide subunits held together by inter-chain interactions, rather than intra-chain bonding within a single backbone.