Question

Why is the molecular formula for a disaccharide not double that of a monosaccharide?

Answer 1

The reason the molecular formula for a disaccharide is not simply double that of a monosaccharide is because when the covalent bond is formed between the carbons of each sugar, an H of one is removed along with an OH from the other. This resulting loss of an H + OH forms H2O (water), therefore the reaction combining the two sugars covalently is called a "dehydration synthesis."
For example, in combining 2 glucose (C6H12O6) molecules, instead of it being 2 × C6H12O6 = C12H24O12, the dehydration synthesis removes 1 H2O to make room for the C-C covalent bond between the two sugars' carbons. Therefore it is 2 H's and 1 O less than that doubling, or: C12H22O11

Answer 2

A molecule of water is lost during condensation two monosaccharides

A disaccharide is formed from a monosaccharide by condensation of two monosaccharides.

During this condensation, a molecule of water is lost. As such, the formula of the disaccharide is less than the sum of (double)the molecular formula of two monosccharide units by H2O due to condensation.

For instance;

C6H12O6 + C6H12O6 --------> C12H22O11

-H2O