Answer:
Total mass of the reactant = 2+2.5 =4.5 mg
Total mass of product = 4.15 mg
therefore, mass of unreacted oxygen = 4.50-4.15 = 0.35 g
Water Is A Liquid And Gravity Pulls Objects Up
I'm pretty sure that significant figures is just the amount of numbers there is. So, in this case I think the answer would be D. 5