Organic Chemistry MCAT Practice Exam

What does mass spectrometry primarily determine?

1. The boiling points of substances

2. The molecular weight of compounds

3. The optical rotation of compounds

4. The color of compounds

The correct answer is: The molecular weight of compounds

Mass spectrometry is a powerful analytical technique used primarily to determine the molecular weight of compounds. The process involves ionizing chemical species and sorting the ions based on their mass-to-charge ratio. This allows the identification of the masses of individual components within a sample, as well as the determination of molecular structures and formulas. In mass spectrometry, the generated ions are detected, and their relative abundance is measured. The resulting mass spectrum displays peaks corresponding to different ions, providing information on the molecular weight and often the structure of the molecules present in the sample. The other options do not align with the primary purpose of mass spectrometry. Boiling points are typically determined using techniques such as distillation or using a boiling point apparatus. Optical rotation, which relates to the ability of a substance to rotate plane-polarized light, is measured through polarimetry. The color of compounds is assessed through methods such as UV-Vis spectroscopy or colorimetric analysis, not mass spectrometry.