What would the percentage of thorium-232 (parent compound) be in a rock that was dated 1.4 billion years?

93.33%

Explanation:

Thorium is a radioactive element with a molecular weight of 232g/mol. Thorium is very stable and can have 14.05 billion years of half-life period. Every half-life passed, the parent compound mass will be decayed by half. If the age of the rock is 1.4 billion years, then the amount of the parent compound will be:r = ½^ (time elapsed/half-life)

r = ½^ (1.4bil years/14.05 bil years)

r = ½^ (0.0996) = 0.9333

r = 93.33%

The rock will have 93.33% of the parent compound and 6.67% of the decayed compound.