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A CAS registry number is separated by hyphens into three parts, the first consisting of up to 6 digits, the second consisting of two digits, and the third consisting of a single digit serving as a checksum. The numbers are assigned in increasing order and do not have any inherent meaning. The checksum is calculated by taking the last digit times 1, the next digit times 2, the next digit times 3 etc., adding all these up and computing the sum modulo 10. For example, the CAS number of water is 7732185, and the checksum is calculated as (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) mod 10 = 105 mod 10 = 5.
Different isomers of a molecule receive different CAS numbers: Dglucose has 50997 and Lglucose has 921608. Occasionally, whole classes of molecules receive a single CAS number: all alcohol dehydrogenases have 9031725.
To find the CAS number of a compound given its name, formula or structure, the following free resources can be used:
See also: NSC number[?], UN number
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