Use the following formula, where
Now, we can substitute the known values.
Now solving for the number of moles present.
Now we can use the formula, where
The answer is rounded to 3 significant digits; as specified by the original question.
Hope this helped :)
I've edited the significant digits in the calculation as the data specified three. Thus the intermediate steps should really carry about one or two intermediate ones. I've carried two to be safe. At the end of the day however, it depends on your instructor or assessment marker.