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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.
Avogadro's number is 6.02*10^23 atoms or molecules per mole. One mole of oxygen is 16 grams, so one mole of oxygen gas is 32 grams. If one mole of oxygen gas is 32 grams, then 16 grams is equal to half a mole of oxygen gas. The number of molecules in half a mole is equal to half of Avogadro's number.