Question

An element has 2 isotopes.The mass of one isotope is A-1 and other is A+2.If the average is A find the isotope A-1.
Please answer...

Answer 1

Let the percentage of isotope having mass (A - 1) be x.

So the percentage of isotope having mass (A + 2) would be (100 - x)

Average atomic mass is given as

$\sf M = \frac{M_1\times percentage\ of\ M_1\ in\ nature + M_2\times percentage\ of\ M_2\ in\ nature}{100}$

Where, M is the average atomic mass and M1 M2 are the mass of isotopes.

So, here we have

A = $\sf\frac{(A - 1)x + (A + 2)(100 - x)}{100}$

⇒ A = $\sf\frac{Ax - x + 100A + 200 - Ax - 2x}{100}$

Cross multiply

⇒ 100A = 200 + 100A - 3x

⇒ 3x = 200

⇒ x = 200/3 ≈ 66.67%

Hence, percentage of isotope (A - 1) is 66.67%