Mastering Chemistry (A website by the textbook maker) has this sameproblem (except the amount of Deuterium is 2.40*10^21 instead of2.50...) They are showing the correct answer as 26.8g ofnaturally occurring Hydrogen gas that must be processed in order toproduce that amount of deuterium. This is half of what your solution provides, as well as what I keep getting for an answer.

Question:
Deuterium, 2H (2.0140 u), is sometimes used toreplace the principal hydrogen isotope 1H in chemical studies.The percent natural abundance of deuterium is 0.015%.
If it can be done with 100% efficiency, what mass of naturally occurring hydrogen gas would have to be processed to obtain asample containing 2.40×1021 2H atoms?