# Question #027ec

##### 1 Answer

You are correct about point **a)**, the mass of the remaining uranium-238 is indeed **1.26 kg**.

Now for point **b)**. The trick here is to know that you don't get **1 gram** of lead-206 for every **1 gram** of uranium-238 that decays, but rather **1 mole** of lead-206 for every **1 mole** of uranium-238 that decays.

Since you're left with **1.26 kg**, the mass of uranium that dcayed is

Convert this value to grams

Now use uranium-238's molar mass to see how many moles decayed

This means of course that you've produced

This translates to a mass of

Rounded to three sig figs, the answer is