In combustion analysis, how do you find moles of C and H?

n(C) = n(CO₂); n(H) = 2 × n(H₂O).

How is oxygen found in combustion analysis?

By difference: total mass minus the masses of C and H.

How do you get the molecular formula from the empirical formula?

Multiply by Mr (from the molecular ion peak) ÷ empirical formula mass.

IR wavenumber range for C=O?

About 1680–1750 cm⁻¹ (strong).

IR appearance of O–H in a carboxylic acid?

Very broad absorption at about 2500–3300 cm⁻¹.

What does the molecular ion peak show?

The relative molecular mass of the compound.

What does an M and M+2 pair of equal height show?

One bromine atom (⁷⁹Br and ⁸¹Br).

What does the number of peaks in a ¹³C NMR spectrum show?

The number of different carbon environments.

State the n + 1 rule.

A proton with n non-equivalent neighbouring protons is split into n + 1 lines.

What does the integration of a proton NMR peak show?

The relative number of hydrogens in that environment.

What does orange to green with acidified dichromate(VI) show?

A primary or secondary alcohol, or an aldehyde, has been oxidised.

What does an orange precipitate with 2,4-DNPH show?

A carbonyl group, in an aldehyde or ketone.

Determining structures from analytical dataEdexcel International A Level Chemistry: Flashcards

Card 1 of 150 of 15 known

Question

How do you find an empirical formula from percentages?

Tap or press Space to reveal

Tap card or press Space to flip

See all 15 cards

How do you find an empirical formula from percentages?: Divide each % by Ar, then divide by the smallest result to get the simplest ratio.
In combustion analysis, how do you find moles of C and H?: n(C) = n(CO₂); n(H) = 2 × n(H₂O).
How is oxygen found in combustion analysis?: By difference: total mass minus the masses of C and H.
How do you get the molecular formula from the empirical formula?: Multiply by Mr (from the molecular ion peak) ÷ empirical formula mass.
IR wavenumber range for C=O?: About 1680–1750 cm⁻¹ (strong).
IR appearance of O–H in a carboxylic acid?: Very broad absorption at about 2500–3300 cm⁻¹.
What does the molecular ion peak show?: The relative molecular mass of the compound.
What does an M and M+2 pair of equal height show?: One bromine atom (⁷⁹Br and ⁸¹Br).
What does an M and M+2 pair in the ratio 3 : 1 show?: One chlorine atom.
What does m/z 43 often represent?: CH₃CO⁺ or C₃H₇⁺.
What does the number of peaks in a ¹³C NMR spectrum show?: The number of different carbon environments.
State the n + 1 rule.: A proton with n non-equivalent neighbouring protons is split into n + 1 lines.
What does the integration of a proton NMR peak show?: The relative number of hydrogens in that environment.
What does orange to green with acidified dichromate(VI) show?: A primary or secondary alcohol, or an aldehyde, has been oxidised.
What does an orange precipitate with 2,4-DNPH show?: A carbonyl group, in an aldehyde or ketone.

Exam questions on Determining structures from analytical data

Compound X contains only carbon, hydrogen and oxygen. Combustion analysis shows that it contains 54.5% carbon and 9.1% hydrogen by mass. The mass spectrum of X has its molecular ion peak at m/z 88.
The infrared spectrum of X has a broad absorption between 2500 and 3300 cm⁻¹ and a strong absorption at 1710 cm⁻¹. State what this shows about the functional group in X.2 marks
Compound Y has molecular formula C₃H₆O. Its mass spectrum has a molecular ion peak at m/z 58 and a base peak at m/z 43. Y forms an orange precipitate with 2,4-dinitrophenylhydrazine but gives no silver mirror with Tollens' reagent.
State how many peaks there are in the ¹³C NMR spectrum of Y, and explain your answer.2 marks
Compound Z contains only carbon, hydrogen and oxygen, and its molecular ion peak is at m/z 74. Complete combustion of 0.370 g of Z gives 0.660 g of carbon dioxide and 0.270 g of water.
Calculate the empirical formula of Z.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).