Chlorine exists in nature as two isotopes. Given the following data calculate the relative abundance of each isotope.
CL = 35.453amu

I just need an example on how to do this. The book is no help!

I'm just guessing on what you are asking here.

Your book probably has a list of the individual isotopes of each element with the mass of each. The mass shown on the Periodic Table is the weighted average of all the isotopes.

(m_1 * pct_1)+(m_2 * pct_2)+...+(m_n * pct_n)

So if an element has an isotope of 106amu, 10% and another isotope of 107amu, 90% - the mass that would be shown on the Periodic Table (if this was not a completely fictitious example) 106.9amu. 106 * .10 + 107 * .90

So all you need to do is determine the percent of each isotope to yield the specified amu.

Hope that helps.

In the question, you are saying that it is 0.4closer to the lighter chlorine and thus must be 0.6 closer to the heavier chlorine. This means that it is in the ratio of 40%:60% or 2:3