Final answer:
Lactose, a disaccharide composed of glucose and galactose, cannot be represented by the formula C6H12O6, unlike the isomeric monosaccharides glucose, galactose, and fructose, which share this chemical formula but differ structurally.
Step-by-step explanation:
The sugar that cannot be accurately represented by the formula C6H12O6 is lactose. Lactose is a disaccharide composed of two monosaccharides: glucose and galactose. Although glucose, galactose, and fructose each individually have the chemical formula C6H12O6 and are classified as isomeric monosaccharides, lactose is not a monosaccharide but rather a larger sugar molecule.
Glucose and galactose are aldoses, which have an aldehyde group, while fructose is a ketose, which has a ketone group. This structural variation occurs despite the three sugars sharing the same molecular formula, which makes them structural isomers or hexoses.