Mass Spectra and Infrared: Identifying an Unknown · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Reading masses and bond fingerprints

Science · Chemistry · ages 17-18
Name ______________________   Date ____________

  1. An IR spectrum shows a broad absorption near 3300. What does it point to?

    • A C=O carbonyl group
    • An O H hydroxyl group
    • A C O single bond only

  2. On a mass spectrum, where do you read the relative molecular mass?

    • Off the tallest peak anywhere
    • Off the highest mass peak, ignoring tiny bumps
    • Off the leftmost peak

  3. A sharp strong absorption near 1700 points to a C=O carbonyl group.

    Circle one:   True   False

  4. A spectrum shows small M+1 and M+2 bumps past the molecular ion. What do they hint at?

    • Broken pump oil in the machine
    • Two extra molecules stuck together
    • Heavier isotopes inside some molecules

  5. Two big peaks sit 15 units apart. What did the ion most likely lose?

    • A hydroxyl group
    • A methyl group
    • A whole benzene ring

  6. A molecular ion sits at 60, with a broad band near 3000 and a sharp one near 1700. Which pair of groups fits?

    • Two benzene rings
    • Alkene and alkyne
    • O H and C=O

  7. A student reads the Mr off the tallest peak in the middle. What is the mistake?

    • Middle peaks never contain carbon
    • Isotope bumps sit in the middle
    • Tallest means most ions, not heaviest molecule

  8. A candidate structure has no C=O, but the spectrum shows a sharp band near 1700. What should you do?

    • Ignore headline peaks entirely
    • Keep it, bands shift anywhere
    • Reject it, it cannot explain the band
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Reading masses and bond fingerprints W1-mt_yotOEg9OTn-s1

  1. An O H hydroxyl group · Broad near 3300 is the hydroxyl fingerprint in the data table.
  2. Off the highest mass peak, ignoring tiny bumps · The whole molecule minus one electron sits at the far right end.
  3. True · That sharp band is the carbonyl fingerprint.
  4. Heavier isotopes inside some molecules · Heavier isotopes make a few molecules one or two units heavier.
  5. A methyl group · A gap of 15 matches the mass of a methyl fragment.
  6. O H and C=O · You need both a hydroxyl and a carbonyl to cover both bands.
  7. Tallest means most ions, not heaviest molecule · Height shows abundance, while position shows mass.
  8. Reject it, it cannot explain the band · A keeper must explain every headline peak it is tested against.
Worksheet · LightMySky