2009 exam

[Mao]

does the first peak in mass spec always show the molar mass of the compound?

Not always. Generally, the peak with the largest m/z is the molar mass, but sometimes you get complete fragmentation and there are no molecular ions. Only sometimes though. (No you don't need to know how or when, just know that you won't ALWAYS get the molecular ion)

Do you know any examples where that happens?

It actually depends on the experimental condition, specifically, the energy of ionising electrons used in the instrument. (So the result can appear differently for different instruments)

[soccerboi]

VCAA 2009 Q2 c
What is the significance of salicylic acid /FeCl3? I don't understand why the examiners report have use these in their explanation and not aspirin?
We are determining purity of the product(aspirin) so why construct a calibration curve of salicylic acid and not aspirin?

[jaydee]

because in uv-visible u need to have a coloured compound or compound that absorbs in uv/visible region. Did u do the aspirin prac and test for salicylic? FeCl3 reacts with salicylic acid to form the purple solution. The more impure it is, the greater amount of salicylic acid in product resulting in deeper shade of purple hence greater absorbance. A pure sample of aspirin would show zero absorbance because it cannot react with FeCl3 to form the purple solution.