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Given that the abundance of isotopes $^{54}Fe, ^{56}Fe$ and $^{56}Fe$ are $5 \%, 90 \%$ and $5 \%$ respectively, the atomic mass of $Fe$ is

$(a)\;55.85 \\ (b)\;55.95 \\(c)\;55.75 \\(d)\;56.05$
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The average isotopic mass or atomic mass = $\sum Mi \times \large\frac{xi}{100}$
Where $m_i$= mass of i th isotope
$x_i$= abundance of ith isotope.
$\therefore $ Atomic mass $=54 \times \large\frac{5}{100}$$+56 \times \large\frac{90}{100}$$+\frac{5}{100}$
$\qquad= 55.95$
Hence b is the correct answer.
answered Mar 21, 2014 by meena.p

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