A molecule of CH3CO2H (acetic acid) contains 1 carbon atom, 4 hydrogen atoms, and 2 oxygen atoms. So, in 300 molecules of CH3CO2H, there will be:
300 * 1 = 300 carbon atoms
300 * 4 = 1200 hydrogen atoms
300 * 2 = 600 oxygen atoms
Therefore, there are 300 carbon atoms in 300 molecules of CH3CO2H.