Accurate Mass and Isotope Patterns as Formula Evidence · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

From an exact mass to a formula

Science · Chemistry · ages 20-21
Name ______________________   Date ____________

  1. A spectrum shows an isotope cluster of roughly nine to six to one. What does it flag?

    • Two chlorine atoms in the molecule
    • No halogens of any kind
    • Exactly one hydrogen atom

  2. Two candidate formulas share one nominal mass but their exact masses differ. How do you choose?

    • Keep the formula whose exact mass matches the accurate measurement
    • Always keep the formula with more atoms
    • Always keep the formula with fewer atoms

  3. Using H about 1 and O about 16, what is the molecular weight of water H2O?

    Answer: ______________

  4. A candidate formula fits the exact mass but its rings and double bonds count cannot match the proposed structure. What now?

    • Accept it anyway, since mass alone is enough
    • Fudge the structure until the count fits
    • Reject the candidate, since all three clues must agree on one structure

  5. An empirical formula weighs 29 and the true molecular mass is 58. By which whole number does every subscript scale?

    Answer: ______________

  6. A neutral molecule has an odd nominal mass. What does the nitrogen rule say?

    • It holds an odd count of nitrogen atoms
    • It holds no nitrogen at all
    • It must be a charged ion, not a neutral molecule

  7. An unknown shows an accurate mass fitting C6H12O6 and C5H8O7 equally well, with no useful isotope pattern. What is the sound next step?

    • Pick one at random, since the masses agree
    • Add the nitrogen rule, which always splits such ties
    • Use the rings and double bonds count plus chemical sense to reject one candidate

  8. A student proposes a formula from an accurate mass plus a nine to six to one cluster, but names zero chlorines. What is the error?

    • Accurate masses can never be trusted for any formula
    • Isotope clusters never identify elements
    • The cluster independently demands two chlorines, so the formula must contain them
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Answer key

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

From an exact mass to a formula W1-mt_68vjdAPNrc-s1

  1. Two chlorine atoms in the molecule · That three peak fingerprint is how the two chlorine isotopes paint a Cl2 group.
  2. Keep the formula whose exact mass matches the accurate measurement · Exact isotope masses make each atom set sum to its own unique total.
  3. 18 · Two hydrogens give 2 and one oxygen gives 16, which add to 18.
  4. Reject the candidate, since all three clues must agree on one structure · The unsaturation count is an independent filter, and failures reject candidates.
  5. 2 · Fifty eight divided by twenty nine is 2, so every subscript doubles.
  6. It holds an odd count of nitrogen atoms · Odd mass points to odd nitrogen count for neutrals built from the common elements.
  7. Use the rings and double bonds count plus chemical sense to reject one candidate · When mass ties, the unsaturation count and structure plausibility cast the deciding vote.
  8. The cluster independently demands two chlorines, so the formula must contain them · Mass and pattern are two independent witnesses, and both must be satisfied at once.
Worksheet · LightMySky