please check my answers for the following questions.

1) when a measurement that has six significant digits is divided by a measurement that has three significant digits, how many significant digits should the answer have?
ANSWER: 2

2) How many hydrogen atoms are in 5 molecules of isopropyl alcohol, C3H7O?
ANSWER: 35

3) How many moles of SO3 are in 2.4 x 10^24 molecules of SO3
ANSWER: 4.0

Number one is incorrect. Three is the answer: in muliplication and division, the answer is limited to the number of significant digits of the least number. In this case, three is the limit in the denominator.

The chemical formula for aspirin is C9H8O4 What is the mass of 0.40 mol of aspirin?
45g
10.8g
160g
72g