Mini ChemistryPart of Mini Education

H3 Chemistry 9813 · Study focus: H3 Chemistry: Interpret mass-to-charge ratio

H3 Chemistry: Interpret mass-to-charge ratio

Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.

Your success criteria

  • Interpret mass-to-charge ratio
  • Use named chemical evidence.
  • Transfer the governing reason to an unfamiliar case.
Diagnose this objective

Read the ratio literally

m/z is ionic mass divided by charge number z.

Nominal mass80 at +1 gives80; mass100 at +2 gives50.

Explore this H3 topic and lesson sequence.

Sum a formula mass

For C6H6+•, 6×12+6×1=78 and z=1, so m/z78.

Use supplied isotope mass numbers rather than average atomic masses for a specified ion.

  1. The same nominal mass78 at z=2 appears at m/z39.

  2. A smaller m/z need not mean a smaller ion.

H3 Chemistry: Interpret mass-to-charge ratio: move from the evidence or givens, through the governing Chemistry idea, to a conclusion that stays inside the selected course boundary.
H3 Chemistry: Interpret mass-to-charge ratio evidence representation. A fixed mass/charge table prevents mass-equals-axis assumptions.
  1. mass80,z1→80; mass100,z2→50; mass128,z2→64.
  2. C6H6 mass78: z1→78, z2→39; 35Cl37Cl+→72.

Text alternative: Text gives every isotope sum, charge divisor and result.

Handle multiple charge

The same nominal mass78 at z=2 appears at m/z39.

A smaller m/z need not mean a smaller ion.

Calculate a singly charged ion

35Cl16O+ has nominal mass51 and z=1.

Write the isotope mass sum before division.

  • Find m/z.
Open the feedback checkpoint after attempting
  • Credit 35+16=51.

Calculate a doubly charged ion

An ion of nominal mass128 with charge2+ occurs at m/z64.

Charge magnitude, not the plus sign alone, is the divisor.

  • Show the division.
Open the feedback checkpoint after attempting
  • Credit 128/2=64.

Start the diagnostic and follow its feedback

Resolve isotope variants

35Cl37Cl+ has mass72; 37Cl2+ has mass74 when both are singly charged.

The isotope label changes numerator, not charge.

  • Assign both positions.
Open the feedback checkpoint after attempting
  • Credit isotope sums72 and74.

Repair mass-equals-axis

m/z equals mass only for z=1.

Always inspect charge notation before assigning a formula.

  • Repair: ‘m/z50 means mass50.’
Open the feedback checkpoint after attempting
  • State mass50 only if singly charged.

Cross-check formula and spacing

Peaks39 and78 can be C6H6²+ and C6H6+•, not two unrelated formulas.

Next: molecular ion at /learning/h3-mass-spectrometry-molecular-ion-lesson.html.

  • Explain the factor-of-two pair.
Open the feedback checkpoint after attempting
  • Credit same mass, charges2 and1.