Copper is listed on the periodic table as having a relative atomic mass of 63.55. Reference books indicate two isotopes of copper, with relative masses of 62.93 and 64.93. What is the percent abundance of each isotope?

Md Ariful Islam

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Md Ariful Islam

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$69%$ for the that weighs 62.93 u and $31%$ for the that weighs 64.93 u.

You can approach this problem by using a single equation; let's say the first isotope contributes to the relative by a fraction $x$ ($x<1$). SInce there are only 2 to consider, the fraction the other isotope contributes with will automatically be $1-x$.

Let's set up the equation

$x * 62.93u + (1-x) * 64.93u = 63.55u$

Solving this for $x$ will produce

$64.93u - 63.55u = 2x -> x = 1.38/2 = 0.69$

Multiplying these fractions (0.69 and 1 - 0.69 = 0.31) by 100% percent to get the results as a percentage will give

$69%$ for the isotpe that weighs 62.93u and
$31%$ for the one that weighs 64.93u.

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