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I know that it is C18H32O16, but how do I derive that from knowing a monosaccharide is C6H12O6?
you have to figure out what is lost when you build the trimer. What could help is to remember the name of the reaction of separation of monomers, starting from the polymer.
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In adition... it might also be a good idea to think about the fact we also have pentoses, hexoses and so on so would be unwise to use numbers if you don't know the monomer.
Consider this too - often polysaccharides arise from dehydration reactions that fuse two monosaccharides. For instance, going from glucose to maltose (a glucose "disaccharide"), you lose 1 O and 2 H's from the molecular formula.
wait. how many waters are lost?
One per bond formed. In other words, you would lose 1 water in forming a disaccharide from two monosaccharides, and 2 waters in forming a trisaccharide from three monosaccharides. However, if you start with a monosaccharide and a disaccharide, you would only lose 1 water when you make a trisaccharide.