The molecular mass of propane-1,2-diol is 76.1 amu . Calculate the molecular mass of propane-1,3-diol, an isomer of propane-1,2-diol.
The Correct Answer and Explanation is:
The molecular mass of propane-1,2-diol is given as 76.1 amu. Now, we need to calculate the molecular mass of propane-1,3-diol, an isomer of propane-1,2-diol.
Molecular Structure
- Propane-1,2-diol (also known as 1,2-propanediol) has the structure:
CH₂OH-CH(OH)-CH₃ - Propane-1,3-diol (also known as 1,3-propanediol) has the structure:
CH₂OH-CH₂-CH₂OH
Both molecules have the same empirical formula (C₃H₆O₂), but differ in the positioning of the hydroxyl (OH) groups along the carbon chain.
Atomic Masses
To calculate the molecular mass of propane-1,3-diol, we need to sum the atomic masses of the constituent atoms:
- Carbon (C): 12.01 amu
- Hydrogen (H): 1.008 amu
- Oxygen (O): 16.00 amu
Step-by-Step Calculation
- Carbon (C): The molecule has 3 carbon atoms, so the mass contribution from carbon is:
3 × 12.01 = 36.03 amu - Hydrogen (H): The molecule has 8 hydrogen atoms, so the mass contribution from hydrogen is:
6 × 1.008 = 6.048 amu - Oxygen (O): The molecule has 2 oxygen atoms, so the mass contribution from oxygen is:
2 × 16.00 = 32.00 amu
Total Molecular Mass
Now, we can add up all the contributions:
36.03 amu (C) + 6.048 amu (H) + 32.00 amu (O) = 74.078 amu
Thus, the molecular mass of propane-1,3-diol is approximately 74.1 amu.
Explanation
Both propane-1,2-diol and propane-1,3-diol have the same molecular formula (C₃H₆O₂), so their molecular masses are very similar. The minor difference in the position of the hydroxyl groups does not change the total molecular mass, which is only a few tenths of a unit lower than propane-1,2-diol’s molecular mass due to the specific arrangement of atoms.
