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1) That looks right. 2) Step 1: Moles=Mass/Mr, so it's 144/72= 2mol. Step 2: Stoichiometry means the ratio of oxygen to pentane is 1:8 so moles of oxygen is moles of pentane times 8. 2*8=16mol of oxygen. Step 3: 16= 9.6*10^24. 3) The ratio of oxygen to carbon in CO2 is 1:2. so it's 192/3 = 64.
Therefore, using atomic percent, it is 1/3 oxygen. However, the mass of oxygen and nitrogen are different, and so the mass percent of oxygen is not 1/3. Nitrogen atoms weigh 14 grams/mole, and oxygen atoms weigh 16 grams per mole, so N2O weighs 44 grams per mole, so the percent oxygen is: 16 Ã· 44 *100% = 36.36% by mass.