0.37 mg Pb-206 0.95 mg U-238 billions-year ago

A rock contains 0.37 mg of Pb-206 and 0.95 mg of U-238.

Approximately how many U-238 atoms were in the rock

when it was formed billions of years ago?

The half-life for U-238 → Pb-206 is 4.5×10^9 yr.

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N_t = 0.95 mg

N_0 = 0.95 mg U + 0.37 mg Pb × [(238 g/mol U) / (206 g/mol Pb)]

= 0.95 + 0.37 × [(238) / (206)]

= 1.3775 mg

(1.3775 mg) (1 g / 1000 mg) (1 mol / 238 g) (6.022×10^23 atoms/mol)

= (1.3775) (1 / 1000) (1 / 238) (6.022×10^23)

= 3.49×10^18 atoms

답: 3.5×10^18 atoms

0.37 mg Pb-206 0.95 mg U-238 4.5×10^9 yr

[키워드] U-238 방사성 붕괴 기준