Starch does not have a set molecular weight; it is a polymer of glucose monomers that are linked together by dehydration reactions. The formula of starch is (C6H10O5)n, where n is the number of glucose monomers linked together.
The monomer has two hydrogens and one oxygen fewer than a typical glucose molecule. This is due to the dehydration reaction between the monomers, which forms a water molecule in the process of forming the bond. Starch is used by plants to hold excess glucose, and in practice, it can hold anywhere from a few hundred to a few hundred thousand glucose monomers.