A group of environmental chemists (Kuhn et al., 2015, Environ. Engineering Science 32(1):54-65) reported on the analysis of natural organic matter (NOM) from Nelson's Creek in the Upper Peninsula of Michigan. The creek water was at a pH of 5.5 and contained 28 mg C/L. Researchers had run a large quantity of water through a reverse osmosis (RO) filter and a cation-exchange column (like a water-softener) designed to keep the RO filters from clogging with CaCO₃. One sample of the NOM they gathered had a Mg content of 8.36 mg Mg/g C. Please assume that all Mg is in the Mg²⁺ form and adsorbed to cation-exchange sites on the NOM. If the NOM is 45.1% C in this sample, then how many cation-exchange sites are occupied by Mg²⁺, in cmol (+)/kg NOM?