Hi there,
1c - a quartet is generated by an ethyl group -CH2CH3 (the two protons of the CH2 group producing the quartet). An ester with molecular formula C4H8O2 must have a CH2CH3 group in it to produce a quartet, so the contenders are methyl propanoate or ethyl ethanoate. In methyl propanoate, the ethyl group is attached to C=O and in ethyl ethanoate it is attached to -O. The chemical shifts should help you to determine which is which.
3b- the molecular formula is C12H4O2Cl4. Ignoring the Cl atoms, the molecular mass is 180. The four chlorines can be 35 or 37 so there are five possible outcomes: 4x35, 3x35 and 1x37, 2x35 and 2x37, 1x35 and 3x37, and finally 4x37.
Therefore, you will have five mass peaks with m/z ratios 320, 322, 324, 326, 328. The relative heights of each m/z peak is determined by the laws of probability. You need to consider each chlorine atom in turn and work out the probability of a molecule having that mass.
For example, if the molecule has a mass of 320 then all four Cl atoms are the 35 isotope. The probability of this is 0.75x0.75x0.75x0.75=0.316.
Now consider if the molecule has a mass of 322. The probability of the first atom being 37 and all the others being 35 is 0.25x0.75x0.75x0.75=0.1055 BUT if atom 2 had a mass of 37 and all the others were 35 this molecule would ALSO have a mass of 322. There are four ways to achieve a mass of 322, each with probability of 0.1055, so the probabilities are added together four times: 0.1055x4=0.42.
The molecule with a mass of 324 would need two 35Cl and two 37Cl. The probability is 0.75x0.75x0.25x0.25=0.035, but there are SIX ways to achieve this. Therefore the probability of the molecule having this mass is 0.035x6=0.21.
Then you have 326: 0.75x0.25x0.25x0.25=0.012. There are four ways for this to happen so the total probability is 0.012x4=0.047.
Finally you have 328: 0.25x0.25x0.25x0.25=0.0039.
Probability total= 0.316+0.42+0.21+0.047+0.0039=1 (allowing for rounding errors). This shows that we haven't missed any possibilities.
The highest peak is m/z 322.
Good luck! Reply if unsure.