The CAS Registry Mumber 16781-33-2 includes 8 digits separated into 3 groups by hyphens. The first part of the number,starting from the left, has 5 digits, 1,6,7,8 and 1 respectively; the second part has 2 digits, 3 and 3 respectively. Calculate Digit Verification of CAS Registry Number 16781-33: (7*1)+(6*6)+(5*7)+(4*8)+(3*1)+(2*3)+(1*3)=122 122 % 10 = 2 So 16781-33-2 is a valid CAS Registry Number.