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7.1 Explain the form of the mass spectrum obtained from bromine, Br2, shown in Figure 7.3. (bromine isotopes and natural abundances: 79Br 50.5% 81Br 49.5%)
Br2 will consist of three isotopomers 81Br81Br, 81Br79Br and 79Br79Br with relative abundances of (very close to) 1:2:1. This gives rise to the parent molecular ion peaks at 162, 160 and 158 and their associated intensities. Fragmentation into atoms will give peaks at m/z or 79 and 81 in the near ratio 1:1.
7.2 Fully interpret the mass spectra obtained from (i) HMn(CO)5, Figure 7.5(b)
Manganese has a single stable isotope with an atomic mass of 55 mu so the parent peak would be expected to be at 55 + (5×28) +1 = 196 mu. This parent peak is observed [HMn(CO)5]+ together with a weaker peak with at 195 mu, which results from the loss of a hydrogen atom to give [Mn(CO)5]+. A series of fragmentation pair peaks are observed resulting from the loss of increasing numbers of CO molecules (−28mu, −56mu, and so on).
7.3 The reaction forming compound A also gives compound B as a side product. CHNSO analysis of B gives 26.65% C, 6.66% H, 10.36% N, 23.71% S and 11.84% O. Determine an empirical formula for B.
The molar masses of C, H, N, S O and Si are 12.01, 1.008, 14.01, 32.06, 16.00 and 28.09 g mol−1, respectively. As no other element is present the residual percentage of silicon in the compound is 100 – (26.65 + 6.66 + 10.36 + 23.71 +11.84) = 20.78 %. The amounts present are, therefore,
n(C)= 
n(H)= 
n(N)= 
n(S)= 
n(O)= 
n(Si)= 
These amounts are in the ratio 3:9:1:1:1:1 giving the empirical formula of C3H9NSOSi for compound B. This product is (CH3)3Si-N=S=O though determining this molecular arrangement would require additional spectroscopic and characterisation techniques.
7.4. Calculate how much MgSO4.6H2O needs to be dissolved in 1 litre of water to make a 10 ppm magnesium standard for AAS analysis.
10 ppm magnesium is equivalent to 10 mg/l and, therefore, sufficient MgSO4.6H2O that contains 10 mg of magnesium needs to be dissolved in 1 litre. The RMM of MgSO4.6H2O is 228.47 gmol−1 and the atomic mass of magnesium is 24.31 gmol−1. So 10 mg of Mg is contained in 10 × 228.47/24.31 mg of MgSO4.6H2O = 93.98 mg.
7.5 Heating hydrated iron (II) sulfate further to 900 °C results in an additional weight loss to 25.2% of the original mass. Explain this observation in terms of the further decomposition of FeSO4
This is the result of decomposition of FeSO4 to FeO with the loss of SO3(g), (RMM 80.06 gmol−1). The percent of the original mass this corresponds to is
(80.06/277.9) × 100 = 28.81 %
using the RMM of FeSO4.7H2O as 277.90 g mol−1
The total weight loss is, therefore, 100 – (45.36 + 28.81) giving a residual 25.8% weight is good agreement with experimentally observed value.