Why is the relative molecular mass of Paracetamol 151, when, if each element is added up, the total is 153.1707?

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Jenkin writes ... The molecular formula of paracetamol is C8H9O2N. If we use the usual approximate relative atomic masses, that is C = 12, H = 1, O = 16, N = 14, then the relative molecular mass is 151. These are the values that are in general use. It is true that the Nuffield Book of Data gives values of relative atomic mass to several decimal places (C = 12.0111, H = 1.0079, O = 15.9994, N = 14.0067). Using these values gives Mr for paracetamol = 151.1654, still 151 to 3 significant figures. I can't figure out how you reach the value 153.1707, but in any case these very precise values need not concern you in ordinary situations such as calculating a % yield. Your own experimental data, such as weighings, are unlikely to justify their use. A further thought on this question. You may have arrived at 153 by thinking that the molecular fomula is C8H11O2N. Remember that the ring carbon atoms attached to the OH and to the NHCOCH3 groups do not also carry a hydrogen atom, so there are ... more
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