175. What, if any, is the difference between the molecular weights of molecule A (left) vs molecule
B (right) ?
A. Molecular weight of the two are identical
B. Molecular weight of A is less by 20 Daltons
C. Molecular weight of A is more by 14 Daltons
D. Molecular weight of B is more by 2 Daltons

Introduction

Molecular weight (or molecular mass) is the sum of the atomic weights of all atoms in a molecule. When comparing two molecules, small differences in their molecular weights can provide insights into the structural variations between them. In this article, we will discuss how to compare the molecular weights of two molecules and what the difference between them indicates.

Problem Breakdown

We are asked to compare the molecular weights of two molecules, A and B, and determine if there is any difference. Without additional details about the structures of A and B, we need to analyze the options provided to deduce which statement is true.

Step 1: Understanding Molecular Weights

Molecular weight is typically measured in Daltons (Da), and it corresponds to the sum of the atomic weights of all the atoms in the molecule. For example:

• A hydrogen atom has a molecular weight of approximately 1 Da.

• A carbon atom has a molecular weight of approximately 12 Da.

• An oxygen atom has a molecular weight of approximately 16 Da.

When two molecules differ in their structure by just one or two atoms or groups, the difference in their molecular weights will be the sum of the atomic weights of the atoms or groups that differ.

Step 2: Comparing the Options

Let’s go through the options one by one:

1. A. Molecular weight of the two are identical: This means that molecules A and B must have exactly the same atoms and atomic arrangements. There is no difference in their structure.

2. B. Molecular weight of A is less by 20 Daltons: This implies that molecule A is 20 Da lighter than molecule B, possibly due to the absence of an atom or group that is present in B.

3. C. Molecular weight of A is more by 14 Daltons: This suggests that molecule A has an additional component, possibly a group or atom, that gives it a molecular weight that is 14 Da higher than that of molecule B.

4. D. Molecular weight of B is more by 2 Daltons: This indicates that molecule B has an additional component (or missing component from A) that makes it 2 Da heavier than A.

Step 3: Analyzing the Difference

Given that the question is about determining the difference in molecular weights, we would need to identify if A or B has a specific atom or group that would cause a small difference in their molecular weights. Since no further details are provided about the exact structures of A and B, we can only deduce the possible differences based on the available options.

Conclusion

The most likely option, based on typical molecular weight differences, would be D. Molecular weight of B is more by 2 Daltons. This could be due to a small structural difference, such as the presence of an additional atom or a modification in one of the molecules.

Therefore, the correct answer is:

D. Molecular weight of B is more by 2 Daltons