To start with, calculate the molar mass of petrol. To do so, we must reference a periodic table with molar masses on it. When we do, we find the molar mass of carbon to be #12.01g# and the molar mass of hydrogen to be #1.008g#. So #(12.01*8)+(1.008*18)=112.224g#
We use that to calculate the amount of moles in the sample by dividing the mass of the sample by the calculated molar mass: #72/114.224# to get #0.63 mol# of petrol.
Next, we must recall Avogadro's Number which is #6.022*10^23#. This represents the amount of molecules per mole. So we multiply this value by the amount of moles in the sample to get the amount of molecules in the sample: #0.63*6.022*10^23=3.8*10^23.# Don't forget that the smallest amount of sig figs in the problem is two with #72g# so that's how many sig figs should be included in your answer.
