Question

Three isotopes of argon occur in nature – 36 18Ar, 38 18Ar, 40 18Ar. Calculate the average atomic mass of argon to two decimal places, given the following relative atomic masses and the abundances of each of the isotopes: argon36 (35.97 amu; 0.337%), argon-38 (37.96 amu; 0.063%), argon-40 (39.96 amu; 99.600%). 1. 119.89 amu 2. 39.95 amu 3. 39.96 amu 4. 35.97 amu 5. None of these 6. 37.96 amu 7. 37.95 amu 8. 35.96 amu

Answer 1

Answer: 3) 39.96 amu

Explanation:

Mass of isotope Ar- 36 = 35.97 amu

% abundance of isotope Ar- 36= 0.337% = $\frac{0.337}{100}=3.37\times 10^{-3}$

Mass of isotope Ar- 38 = 37.96 amu

% abundance of isotope 2 = 0.063 % = $\frac{0.063}{100}=6.3\times 10^{-4}$

Mass of isotope Ar- 40 = 39.96 amu

% abundance of isotope 2 = 99.600 % = $\frac{99.600}{100}=0.996$

Formula used for average atomic mass of an element :

$\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}\times {{\text { fractional abundance}})$

$A=\sum[(35.97\times 3.37\times 10^{-3})+(37.96\times 6.3\times 10^{-4})+(39.96\times 0.996)]$

$A=39.96amu$

Therefore, the average atomic mass of argon is 39.96 amu