H3 Chemistry 9813 · Study focus: H3 Chemistry: Use peak integration to count protons
H3 Chemistry: Use peak integration to count protons
Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.
Your success criteria
- Use peak integration to count protons
- Use named chemical evidence.
- Transfer the governing reason to an unfamiliar case.
Area gives a ratio
A C4H8O spectrum has integral areas 30, 20 and 30 units. Divide by 10 to obtain 3:2:3, totalling eight protons.
Integration is area under a signal/multiplet, not its tallest line.
Integral and scaling
Integrated area is proportional to the number of equivalent protons contributing to a signal.
Convert areas to the simplest ratio, then scale to a known total or candidate molecular formula.
For areas 1.5:1.0:1.5, divide by 0.5 to get 3:2:3. Confirm the sum equals the eight hydrogens available.
Exchangeable OH/NH integrals can be unreliable; use supplied data and other evidence.
| Case | δ / ppm | Measured area | Common divisor | Normalised ratio | Assigned H | Checksum |
|---|---|---|---|---|---|---|
| C₄H₈O | 2.1 | 30 | 10 | 3 | 3 | 3 + 2 + 3 = 8 H |
| C₄H₈O | 2.5 | 20 | 10 | 2 | 2 | 8 H total |
| C₄H₈O | 1 | 30 | 10 | 3 | 3 | 8 H total |
| area 1:2; total 6 | — | 1:2 | sum=3; scale ×2 | 1:2 | 2 H:4 H | 6 H |
| area 4.8:3.2:1.6 | — | 4.8:3.2:1.6 | 1.6 | 3:2:1 | 3 H:2 H:1 H | 6 H |
Text alternative: Every bar length is duplicated by raw numeric area, divisor, normalised ratio and proton count.
Use totals as a checksum
For areas 1.5:1.0:1.5, divide by 0.5 to get 3:2:3. Confirm the sum equals the eight hydrogens available.
Exchangeable OH/NH integrals can be unreliable; use supplied data and other evidence.
Scale 1:2 to six H
Let counts be k and 2k. Since 3k=6, k=2.
The two environments contain 2H and 4H.
- Scale an area ratio 1:2 to a six-proton total.
Open the feedback checkpoint after attempting
- Credit algebra or proportional reasoning to 2H:4H.
Read an ethyl ester
Integrals 3:2:3 match acyl CH3, OCH2 and terminal CH3 in ethyl ethanoate.
Use chemical shift and splitting to decide which 3H is which.
- Assign only the proton counts from 3:2:3.
Open the feedback checkpoint after attempting
- Credit 3H, 2H and 3H; do not use area to claim neighbours.
Normalise decimal areas
Areas 4.8, 3.2 and 1.6 share factor 1.6.
The ratio is 3:2:1 and totals six protons.
- Normalise 4.8:3.2:1.6 and check a C3H6O candidate.
Open the feedback checkpoint after attempting
- Credit 3:2:1 and total six.
Height is not area
A narrow line can be taller than a broad line with the same or smaller area.
Use the integral trace/value supplied across the entire multiplet.
- Repair: “The tallest signal contains the most protons.”
Open the feedback checkpoint after attempting
- Replace height with integrated area and keep multiplicity separate.
Count, scale, then assign
Complete twelve integral checks, then unseen aromatic ketone assessment and different diol re-test.
Next objective: interpret spin–spin splitting at /learning/h3-nmr-spin-spin-splitting-lesson.html.
- Show the scaling of 30:20:30 for C4H8O.
Open the feedback checkpoint after attempting
- Credit simplest ratio 3:2:3, eight-proton checksum and bounded structural use.