The mass spectrum of a rock sample shows that it contains 4.40 g 238U and 1.21 g

The mass spectrum of a rock sample shows that it contains 4.40 μg 238U and 1.21 μg 206Pb. Assuming that all of the lead was produced from radioactive decay of uranium-238, how old is the rock? The half-life of 238U is 4.5 × 10⁹ years.

Strategy We need to calculate the original mass of uranium, which is equal to the 4.40 μg present, plus the mass that decayed into 1.21 μg 206Pb. The original mass, the current mass,

and the half-life will allow us to calculate the age of the rock sample.