CHN Calculator
https://xraytools.com/chncalc?formula=C9H8O4
The molecular weight a formula implies and the elemental analysis it predicts — the percentages a combustion analysis is compared against.
- You supply
- A chemical formula. Brackets nest and may carry a fractional multiplier (
Cu(NO3)2,K3[Fe(CN)6],(SiO2)0.5), and a charge may be written after a caret (SO4^2-) or as a bare sign (Na+). Adducts and hydrates are written with a dot or an asterisk, with the count in front of the part it multiplies (CuSO4·5H2O,KAl(SO4)2*12H2O); a full stop is a decimal point here and not a separator, sinceCuSO4.5H2Ois also a formula with a count of 4.5 in it. The charge is noted and left out of the arithmetic: every quantity here is a sum over atoms, and an electron is one part in 105 of a carbon. - Reading it
- Every element in the formula is reported, not only C, H and N, so a CHNS analysis is covered by the same table. Percentages are by mass, which is how elemental analysis is reported. An unrecognised element symbol is refused rather than dropped, because dropping it would renormalise the rest and still look right.
Worked examples: aspirin · a nickel complex · copper nitrate · blue vitriol
See also: Absorption Coefficient Calculator · Isotope Pattern · Crystal Density Calculator
Input
Results
| Formula as entered | C9H8O4 | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Read as | C9H8O4 | ||||||||||||
| Elemental contributions |
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| Molecular weight | 180.157(5) g/mol | ||||||||||||
| Elemental analysis |
The ± figures come from the uncertainty of the atomic weights themselves. They are strongly correlated and must not be added up — the same atomic weight enters every row, so the total is exactly 100 % whatever the weights turn out to be. |
The ± figures are standard uncertainties derived from the IUPAC standard atomic weights — the spread of normal terrestrial materials, not the precision of a measurement. Where the weight is published as an interval, the figure here is its half-width divided by , which is what a rectangular distribution over that interval has as its standard deviation — carbon's [12.0096,12.0116] is half-width 0.001 and enters as 0.00058, which is that over . Without knowing which of the two a figure is, it cannot be propagated at all. For an ordinary organic compound they are a few thousandths of a percentage point, far below what a combustion analysis can resolve, so they decide how many digits are worth printing rather than whether a sample matches. Where an element's weight is published as an interval, the value used here is its midpoint, which is why carbon enters as 12.0106 rather than the abridged 12.011 a textbook table prints. The two differ in the fourth decimal and the midpoint is the one that carries the interval it came from.
Where this comes from
- Atomic weights of the elements 2013 (IUPAC Technical Report)
J. Meija et al., Pure Appl. Chem. 2016, 88, 265–291 · doi:10.1515/pac-2015-0305
The standard atomic weights the molar mass is built from, and the reason some of them are published as an interval rather than a value: the weight depends on where the sample came from. - Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)
A. Possolo, S. van der Veen, J. Meija and D. B. Hibbert, Pure Appl. Chem. 2018, 90, 395–424 · doi:10.1515/pac-2016-0402
How an interval becomes a standard uncertainty and how it propagates into a molar mass, which is what the ± column does. Its worked carbon dioxide example is reproduced digit for digit by this site’s own checks.