Naturally occurring chlorine is a mixture of two isotopes with mass number 35 and 37. Theisotope with mass number 35 is three times as common as the isotope with mass number 37. Naturally occurring bromine is a mixture of two isotopes with mass numbers 79 and 81. They are present in equal amounts.What fraction of the naturally occurring compound CH2BrCl has a relative molecular mass of 128?

Answer:

26) C

We are not sure which isotopes of chlorine and bromine are present in the molecule.

Let Ar of Br in the molecule = M
Let Ar of Cl in the molecule = N
Mr of CH₂BrCl = 128

128 = (12 x 1) + (1 x 2) + (M x 1) + (N x 1)
128 = 12 + 2 + M + N
114 = M + N

The only possible combination of isotopes that can be present in the molecule is: Cl = 35 and Br = 79

Chlorine isotope 35 is three times more common as isotope 37.
Ratio = 3 : 1
Total parts = 3 + 1 = 4
So probability of having isotope 35 in the molecule = ³⁄₄

Bromine isotope 79 is equally as common as isotope 81.
Ratio = 1 : 1
Total parts = 1 + 1 = 2
So probability of having isotope 79 in the molecule = ¹⁄₂

Probability of having Chlorine isotope 35 and Bromine isotope 79 = ³⁄₄ x ¹⁄₂ = ³⁄₈