I am trying to calculate a chemical formula. How do I calculate the empirical formula when ash is present in the percentage composition?


  • C- 47.7%
  • H- 7.2%
  • N- 1.8%
  • S- 0.9%
  • O- 38.0%
  • Ash- 4.4%

Since the ash is present at 4.4%, which could not be added into a chemical formula, should I exclude it?

If yes, should I just use the current percentage composition and neglect ash OR find the new percentage composition based on the sum of 95.6 g of other compositions?

For context, above is the composition of wastewater, where it is almost impossible to accurately obtain the composition of ash due to multiple components and mixtures available in it.

How do I solve this issue?

  • $\begingroup$ Our university microanalysis lab used to see this problem. It was due to students submitting samples that were contaminated with scraped off filter paper. ;-) $\endgroup$
    – Ed V
    Commented Apr 22, 2022 at 11:28
  • $\begingroup$ You cannot, unless you know the ash composition. $\endgroup$
    – Poutnik
    Commented Apr 22, 2022 at 12:30
  • $\begingroup$ Does the ash come from an external source? If so, ignore it and just compute on what's left. That makes some pretty strong assumptions though... $\endgroup$
    – Zhe
    Commented Apr 22, 2022 at 20:59


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