Lactose is a disaccharide composed of one glucose monomer and one galactose monomer. The molecular formula for both glucose and galactose is C6H12O6.Which of the following shows the molecular formula for lactose?

A glucose molecule and a galactose molecule together have the formula C12H24O12: add their atom counts, 6+6 carbons, 12+12 hydrogens, and 6+6 oxygens. When the two monosaccharides join to form lactose, they lose one water molecule, H2O. Subtracting H2O gives C12H22O11, which is choice D.

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