Chemistry 2004 JAMB Past Questions - Question 12: Chlorine consisting of two isotopes of mass numbers 35 and 37 in the ratio 3:1 has an atomic mass of 35.5. Calculate the relative abundance of the isotope of mass number 37.

Correct Answer

B

Explanation

total ratio = 3+1 = 4chlorine 35 = 3, chlorine 37 = 1therefore the relative abundance of isotope 37 = 1 /4 x 100 /1 = 25
To calculate the relative abundance of the isotope with mass number 37, we can use the following formula:

Average atomic mass = (mass1 * abundance1) + (mass2 * abundance2)

Given that the average atomic mass of chlorine is 35.5, and the isotopes have mass numbers 35 and 37 in the ratio 3:1, we can set up the following equation:

35.5 = (35 * 3x) + (37 * x)

Where x represents the relative abundance of the isotope with mass number 37.

Solving for x:

35.5 = 105x + 37x
35.5 = 142x
x = 35.5 / 142
x = 0.25

So, the relative abundance of the isotope with mass number 37 is 0.25, or 25%.