Q.Sugars are technically called carbohydrates, referring to the fact that their formulae are only multiple of C(H2O). Hexoses therefore have six carbons, twelve hydrogens and six oxygen atoms. Glucose is a hexose. Choose from among the following another hexose.

Monosaccharide Classification

Sugars earn the name "carbohydrate" quite literally: their molecular formulae are simple multiples of the unit C(H2O) — carbon, as if bonded to water. That one fact is the key to the entire classification system for monosaccharides, because once you know a sugar's carbon count, you already know its molecular formula, and once you know its functional group, you know what chemical family it belongs to.

Classifying by carbon count — the tri-/tetr-/pent-/hexose ladder

Every monosaccharide can be placed on a simple ladder based on how many carbon atoms it has:

A classic exemplar trap gives you glucose (a hexose) and asks you to pick out another hexose from a list that also includes ribose, ribulose and erythrose. The temptation is to reason about sweetness or familiarity — but the only thing that matters is the carbon count. Ribose and ribulose are pentoses (5 carbons, ruled out), erythrose is a tetrose (4 carbons, ruled out), and only fructose shares glucose's six-carbon hexose formula.

Classifying by functional group — aldose versus ketose

Carbon count isn't the only axis of classification. Monosaccharides also split into two families based on which functional group they carry:

• Aldoses — carry an aldehyde group. Glucose is the classic example.
• Ketoses — carry a ketone group. Fructose is the classic example.

Why this classification matters beyond naming …