Potassium- 40 has a half-life of 1 point 2 7 7 times 10 to the 9th power years. After 1 point 0 2 2 times 10 to the 10th power years, how much potassium- 40 will remain from a 500 point 3 - g sample?

The time elapsed is about 8 half-lives because (1.022 × 10^10) ÷ (1.277 × 10^9) ≈ 8. After 8 half-lives, the remaining amount is 500.3 × (1/2)^8 = 500.3 ÷ 256 ≈ 1.95 g. So approximately 1.95 g remains, choice B.

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