Question

Use the data given in the following table to calculate the molar mass of naturally occuring argon isotopes:

Isotope	Isotopic molar mass	Abundance
36Ar	35.96755 g mol–1	0.337%
38Ar	37.96272 g mol–1	0.063%
40Ar	39.9624 g mol–1	99.600%

Answer 1

Given:

Naturally occurring isotopes of argon and their data: 

Isotope	Isotopic molar mass (g mol−1)	Abundance (%)
36Ar	35.96755	0.337
38Ar	37.96272	0.063
40Ar	39.9624	99.600

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Step 1: Convert percentage abundance into fractional abundance

³⁶Ar = 0.337 / 100 = 0.00337 
³⁸Ar = 0.063 / 100 = 0.00063 
⁴⁰Ar = 99.600 / 100 = 0.99600

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Step 2: Calculate average molar mass of argon

Average molar mass = Σ (Isotopic mass × Fractional abundance)

= (35.96755 × 0.00337) + (37.96272 × 0.00063) + (39.9624 × 0.99600)

= 0.1212 + 0.0239 + 39.8026

= 39.95 g mol⁻¹

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Final Answer:

The molar mass of naturally occurring argon is: 
39.95 g mol⁻¹