### What is the CAS number?

**CAS number format**

A CAS number is divided into three parts with a hyphen "-". The first part has 2 to 6 digits, the second part has 2 digits, and the third part has 1 digit as a check code. CAS numbers are in ascending order and have no intrinsic meaning. The verification code is calculated as follows: Multiply the last digit of the CAS sequence number (the first and second digits) by 1, the last second digit by 2, and so on. Then add all the products together and divide the sum by 10. The remaining digit is the third digit. For example, if the first two CAS numbers of water (H2O) are 7732-18, its verification code is (8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6) mod 10 = 105 mod 10 = 5. (mod is the complementary operator)

**Error CAS number Example:**

The common format of the CAS number is XXXXXXX-XX-X. If appear

A. 0000050-00-0. The first digit 0000050-XX-X in A cannot start with an integer 0. The correct notation is 50-XX-X.

B, 50-0-0, the second digit in B, 50-0-X, contains two digits, and is correctly written as 50-00-X.

C, 50-00-1, the third digit in B is the check digit 50-00-1. The third digit is a check digit, which is calculated by the check digit. The correct notation is 50-00-0.