OCR A-Level Chemistry: Chapter 17 - Spectroscopy
This flashcard set explains the core concepts of mass spectrometry, including detection of mass-to-charge ratios, formation of molecular ions, and interpretation of peaks. It also covers isotopic contributions and fragmentation patterns used to analyze molecular structure.
What does the mass spectrometer detect?
The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound.
Key Terms
What does the mass spectrometer detect?
The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound.
How are molecular ions formed in mass spectrometry?
Electron is lost to form a positive ion
How is molecular mass found from a mass spectrum?
The molecular ion peak (M+ peak) has to be located.
What is the cause of the M + 1 peak?
Some carbon is present as the carbon-13 isotope.
What is fragmentation?
The molecular ions break down into smaller pieces called fragments.
What causes the other peaks in a mass spectrum?
Fragment ions
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| Term | Definition |
|---|---|
What does the mass spectrometer detect? | The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound. |
How are molecular ions formed in mass spectrometry? | Electron is lost to form a positive ion |
How is molecular mass found from a mass spectrum? | The molecular ion peak (M+ peak) has to be located. |
| Some carbon is present as the carbon-13 isotope. |
| The molecular ions break down into smaller pieces called fragments. |
What causes the other peaks in a mass spectrum? | Fragment ions |
How can a mass spectrum differentiate between structural isomers? | They will break apart differently and thus will have different mass spectra. |
What is the effect on a bond when it absorbs IR? | Bend or stretch more as they gain energy. |
What does the amount a bond streches or bends depend on? | Mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms |
What is unique about each bond with regards to IR absorption? | Can only absorb radiation that has the same frequency as the natural frequency of the bond. |
| Wavenumber 200/cm to 4000/cm |
How does infrared spectroscopy work? | Sample placed inside an IR spectrometer, |
| Peaks |
What do peaks help us predict? | The bond types in the molecules. |
What is below 1500/cm known as on the IR spectrum and why? | Fingerprint region The peaks in this region are unique to each molecule. |
In an exam question what can be used to determine the molecule in question? | IR absorptions on formula sheet |
What is the typical sequence for identification of an unknown substance? | Elemental analysis - determine empirical formula |