The half-life of Potassium-40 is about 1.25 billion years. This means that after each half-life, the number of moles of Potassium-40 in the rock will be halved. Using this information, we can calculate the answers to the questions as follows:
i. After the rock aged 1.25 billion years, half of the original number of moles of Potassium-40 would have decayed. This means that there would be 8 moles of Potassium-40 remaining in the rock.
ii. After the rock aged an additional 1.25 billion years, another half-life would have passed, and half of the remaining 8 moles of Potassium-40 would have decayed. This means that there would be 4 moles of Potassium-40 remaining in the rock at present day (2.50 billion years + 1.25 billion years + 1.25 billion years = 3 half-lives).
Therefore, the number of moles of Potassium-40 in the rock would be 8 after 1.25 billion years and 4 after an additional 1.25 billion years (present day).