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Question Video: Determining the General Molecular Formula of a Monosaccharide and Working Out the Molecular Formula of a Disaccharide Composed of Two Molecules of a Given Monosaccharide Biology

The structure of monosaccharides and disaccharides can be determined using general formulas. a) What is the general molecular formula of a monosaccharide? b) A monosaccharide has the formula C₆H₁₂O₆. Two of these monosaccharides join by a condensation reaction. What is the molecular formula of the resulting disaccharide?

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Video Transcript

The structure of monosaccharides and disaccharides can be determined using general formulas. What is the general molecular formula of a monosaccharide? A monosaccharide has the formula C6H12O6. Two of these monosaccharides join by a condensation reaction. What is the molecular formula of the resulting disaccharide?

This question is asking us to recall what we know about the chemical formulas of monosaccharides and disaccharides, which are both types of carbohydrates. Let’s just focus on the first part of this question for now. Here, we’re interested in the general molecular formula of a monosaccharide. And since it’s the general molecular formula, it should be able to apply to more than one type of monosaccharide. Let’s also take a moment to recall that mono- is a word part that means one and saccharide means sugar. So we’re looking for the general molecular formula of one sugar.

We know that sugars are a type of carbohydrate, and a great clue to their chemical formula is present right here in the name. Carbo- is a word part that refers to carbon and hydrate means water. So we know that these molecules consist of some carbon and some H2O. A monosaccharide that you’re likely familiar with already is glucose. Glucose is the simple sugar used by ourselves in cellular respiration. One molecule of glucose looks something like this and has the chemical formula C6H12O6 because glucose possesses six atoms of carbon, 12 atoms of hydrogen, and six atoms of oxygen.

So, like our carbohydrate ratio, there are twice as many atoms of hydrogen as there are carbon and oxygen. But glucose has six times the atoms of our base ratio. So in order to complete our general formula, we must multiply the entire ratio by a variable, which we’ll call 𝑛. Continuing our example, when 𝑛 is equal to six, that means we have six times the carbon, hydrogen, and oxygen atoms in our molecule, which comes out to C6H12O6. And this is the correct chemical formula for our example monosaccharide, glucose. So the general molecular formula for a monosaccharide is CH2O𝑛.

Now let’s take a look at the second part of this question. It’s asking us about the resulting disaccharide molecule that forms when two monosaccharides join. Di- is a prefix that means two. So let’s take a look at what happens when two monosaccharides bond together to form a disaccharide. We’ll use glucose as the monosaccharides in our example, since they have the chemical formula C6H12O6 and we’re already familiar with their structure.

When glucose bonds with glucose, the disaccharide that’s formed is called maltose. During the condensation reaction, two of the hydroxyl groups are rearranged so that both molecules are now sharing one oxygen atom. This process releases two hydrogen and one oxygen atoms, and they bond to form one molecule of water. The fact that a molecule of water is released is how the condensation reaction gets its name. So to figure out the chemical formula, we could count up the number of atoms in our maltose molecule. Or we can recall that that maltose molecule was made by joining two molecules of glucose together and releasing a water molecule in the process. Either way, you’ll come up with the same solution. The molecular formula for the resulting disaccharide is C12H22O11.

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