It makes no sense to speak of a defined molecular mass for CMC, since it is polydisperse. Thus, concentration is usually given as mg/ml. What would make more sense would be to specify the molarity of glycosidic bonds, calculated from the mass of the protomers (152 Da) + that of the carboxymethyl substituant multiplied by the degree of substitution (59 Da * 0.4). Even like that, the calculation is somewhat arguable, since not all glycosidic bonds are accessible to hydrolysis because of steric hindrance due to carboxymethyl substitution

Dear Christian,

The average molecular weight of carboxymethyl cellulose (CMC) (CAS Number 9004-32-4) has been reported 250000 g/mol. If you know the exact CAS Number of the purchased CMC, you can search to find its exact molecular weight. Otherwise, for an approximate calculation of the concentration you can use the mentioned molecular weight to calculate its concentration base on mmol/ml.