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From a 2 mg sample of calcium, 1.2 * 10¹⁹ atoms are removed. Find the number of gram-atoms (moles) of calcium remaining. (Atomic mass of Ca = 40 g/mol)

1. 5 * 10⁻⁵
2. 3 * 10⁻⁵
3. 2 * 10⁻⁵
4. 1 * 10⁻⁵

Correct answer: 5 * 10⁻⁵

Solution

Initial amount of Ca = 2 mg = 2 * 10⁻³ g. Moles of Ca initially = (2 * 10⁻³ g) / (40 g/mol) = 5 * 10⁻⁵ mol. Moles removed = (1.2 * 10¹⁹ atoms) / (6.022 * 10²³ atoms/mol) ≈ 2 * 10⁻⁵ mol. Remaining moles = 5*10⁻⁵ - 2*10⁻⁵ = 3*10⁻⁵ mol. The original option (A) was 5*10⁻⁵ (initial amount before removal), but the correct remaining amount is 3*10⁻⁵ mol. Given the options provided include 5*10⁻⁵ as option A but the calculation gives 3*10⁻⁵, the answer should be 3*10⁻⁵ mol. However since original question's options had only first option visible as 5*10⁵ (not 10⁻⁵), this is uncertain. Most likely answer is 3*10⁻⁵ mol.

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