# chemistry aqa a-level question help

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#1
Chemical analysis shows that a chlorofluoroalkane, B, contains by mass 51.6% fluorine, 32.1% chlorine and no hydrogen.
35 37
Chlorine exists as two isotopes, Cl and Cl, in the ratio 3:1
19 Fluorine only exists as one isotope, F.
A mass spectrum of B is obtained using electron impact ionisation. The mass spectrum shows three molecular ion peaks at m/z = 220, 222 and 224.
Determine the formula of each of the three molecular ions of B. Predict and explain the ratio of the relative abundancies of each of the
three molecular ion peaks at m/z = 220, 222 and 224. To gain full marks you must show all your working.
[6 marks]

ive worked out the empirical formula but idk where to go from there also can someone explain why the ratios of chlorine are relevant
0
1 month ago
#2
So the formula of the compound is C3Cl2F6
However as chlorine exists as two isotopes, there will be different masses for this compound, depending on which isotopes of chlorine it contains. As the chlorine isotopes are in the ratio 3:1 it means 3 quarters of the isotopes will be Cl35 and one quarter will be Cl37. Each molecule contains two chlorine atoms meaning there are three different combinations of chlorine istopes, Cl35 and Cl35, Cl35 and Cl37 (or Cl37 and Cl35), and Cl37 and Cl37. Using the fractions of the abundance of each chlorine isotope you can make a probability tree to find the ratio of each combination. Cl35 and Cl35 give a total molecular mass of 220 and using the probability tree have a probability of 9/16. Cl35 and Cl37 give a molecular mass of 222 and has a probability of 6/16. Cl37 and Cl37 give a total molecular mass of 224 and have a probability of 1/16. So the ratio of abundances is 9 1. Here's a picture of the probability tree I used, I hope this helps  0
#3
(Original post by Megan4125)
So the formula of the compound is C3Cl2F6
However as chlorine exists as two isotopes, there will be different masses for this compound, depending on which isotopes of chlorine it contains. As the chlorine isotopes are in the ratio 3:1 it means 3 quarters of the isotopes will be Cl35 and one quarter will be Cl37. Each molecule contains two chlorine atoms meaning there are three different combinations of chlorine istopes, Cl35 and Cl35, Cl35 and Cl37 (or Cl37 and Cl35), and Cl37 and Cl37. Using the fractions of the abundance of each chlorine isotope you can make a probability tree to find the ratio of each combination. Cl35 and Cl35 give a total molecular mass of 220 and using the probability tree have a probability of 9/16. Cl35 and Cl37 give a molecular mass of 222 and has a probability of 6/16. Cl37 and Cl37 give a total molecular mass of 224 and have a probability of 1/16. So the ratio of abundances is 9 1. Here's a picture of the probability tree I used, I hope this helps  thank you so so much i now understand how to do it thanks to you 0
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