Nuclidic molecular mass

The molecular mass of a compound is the average mass (in u) of a molecule, weighted among the various isotopic forms (isotopomers) of the different component elements. A nuclidic molecular mass may be defined for a molecule made up of particular nuclides by adding nuclidic atomic masses in the same way that the molecular mass is computed from average atomic masses. 

Find the formula of an organic compound whose dominant nuclidic species has a precise nuclidic molecular mass of 44.025 u, given that no other elements than C, H, O, and N are present and only 12C, H, 160, and 14N are involved. 

Considering all possible combinations of C, H, O, and N, we find six molecular formulas with nuclidic molecular masses of about 44. These are listed in Table 2-7 with nuclidic molecular masses. 

The dominant peak in the mass spectrum of an organic compound gave a nuclidic molecular mass of 117.090 u. If it was known in advance that the compound contained only C, H, and O and that no more than four oxygen atoms are present in the molecule, what is its molecular formula ... 

FORMULA FROM PRECISE NUCLIDIC MOLECULAR MASS... 

In all the following problems the nuclidic molecular mass reported is that of the species containing the most prevalent nuclide of each of its elements. 

An organic ester (an organic salt) was decomposed inside a mass spectrometer. An ionic decomposition product had the nuclidic molecular mass 117.090. What is the molecular formula of this product if it is known in advance that the only possible constituent elements are C, O, and H, and that no more than 4 oxygen atoms are present in the molecule ... 

An intermediate in the synthesis of a naturally occurring alkaloid had a mass-spectrometrically determined nuclidic molecular mass of 205.147. The compound is known to have no more than 1 nitrogen atom and no more than 2 oxygen atoms per molecule, (a) What is the most probable molecular formula of the compound (b) What must the precision of the measurement be to exclude the next most probable formula ... 

Ans. (a) C43ONH19 (nuclidic molecular mass is 205.147). (b) The next closest molecular mass is 205.159 for... 

Column (2) lists the mass numbers of the (C, O, N) skeletons. Column (3) lists the number of hydrogen atoms needed to bring the mass number of the molecule to 44. Column (4) lists the maximum number of H atoms consistent with rules for molecular structure as discussed in Chapters 9 and 15. One such rule is that (H, max) is equal to twice the number of carbon atoms plus the number of nitrogen atoms plus 2. Column (5) lists the allowed formulas consistent with the total mass number and with all the assumptions and rules. Note that all skeletons for which the number in column (3) (the mass shortage to be made up by hydrogen) exceeds the number in column (4) (the amount of hydrogen allowable for the skeleton by the rules of valence) are rejected. Column (6) tabulates the nuclidic molecular masses for the allowed formulas, computed from the nuclidic masses in Table 2-1. When the computed molecular masses are compared with the experimental value, 44.025, it is seen that C2OH4 is the only allowable formula that fits the data within the claimed precision therefore this must be the formula of the substance. 

An alkaloid was extracted from the seed of a plant and purified. The molecule was known to contain I atom of nitrogen, no more than 4 atoms of oxygen, and no other elements besides C and H. The mass-spectrometrically determined nuclidic molecular mass was found to be 297.138. a) How many molecular formulas are consistent with mass number 297 and with the other known facts except the precise molecular weight (b) What is the probable molecular formula ... 

Arts, (a) C13ONH19 (nuclidic molecular mass is 205.147). (h) The next closest molecular mass is 205.159 for C)40H2i. The range of uncertainty in the experimental value should not exceed half the difference between 205.147 and 205.159, i.e., it should be less than 0.006, or about 1 part in 35000. 

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