A Level Chemistry OCR Practice Exam

In mass spectrometry, what is a molecular ion?

1. The ion formed when a molecule gains an electron

2. The positive ion formed when a molecule loses an electron

3. The ion formed by the fragmentation of a molecule

4. The ion that remains unchanged after ionization

The correct answer is: The positive ion formed when a molecule loses an electron

A molecular ion in mass spectrometry refers specifically to the ion that is produced from a molecule when it loses an electron, resulting in a positively charged ion. This process allows the mass spectrometer to measure the mass-to-charge ratio of the original molecule, thereby providing crucial information about its structure and composition. The formation of the molecular ion typically occurs at the initial stage of ionization, where the intact molecule is ionized before any fragmentation takes place, making it an essential part of mass spectrometric analysis. A key aspect of the molecular ion is that it corresponds to the entire molecule's mass minus the mass of the lost electron, which is why it plays a significant role in the identification of the molecular weight of compounds. This ion can be seen in mass spectra usually at the highest m/z value for a given molecular composition, reflecting its functionality in elucidating the molecular formula of the analyte. Other options, while related to ion formation in mass spectrometry, do not accurately define the molecular ion. The correct depiction of the molecular ion as a positive ion results from the loss of an electron underscores its importance in mass spectrometry.