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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.
A mole is a quantity (in grams) that equals the molecular number of the substance. Oxygen has atomic number 16, hydrogen 1, so H2O = 2*1+16=18. So indeed a mole of water is 18grams. The mole (animal) is something like a rat. The qustions (1, 2, 3) you have are rather like quizes, they don't really have answers.