Question 1

2,2-dimethylbutane gives a significant fragment peak at m/z = 57.Which of the following is the most probable structure of the fragment ion?  &#10;A)I&#10;B)II&#10;C)III&#10;D)IV

Accepted Answer

The significant fragment peak at m/z = 57 indicates the loss of a methyl group (15 amu) and the formation of a secondary carbocation on the remaining 4-carbon chain. The most stable secondary carbocation is the one located at the center of the chain (IV), which explains why it is the most probable structure of the fragment ion.

Question 2

What is the molecular ion,m/z,for the following molecule?  &#10;A)72&#10;B)70&#10;C)68&#10;D)66

Accepted Answer

70

Question 3

What is the molecular ion,m/z,for the following molecule?  &#10;A)72&#10;B)70&#10;C)68&#10;D)66

Accepted Answer

72

Question 4

Which of the following statements about the base peak of a mass spectrum is always true?&#10;A)The base peak corresponds to the molecular ion.&#10;B)The base peak corresponds to the most abundant ion.&#10;C)The base peak corresponds to the lowest m/z.&#10;D)None of these.

Accepted Answer

The base peak in a mass spectrum corresponds to the most abundant ion, meaning the ion with the highest intensity in the spectrum. It is not necessarily the molecular ion (A), which is the ion corresponding to the unfragmented parent molecule, and it is not necessarily the lowest m/z (C). Therefore, the correct choice is B.

Question 5

Heptane,C₇H₁₆,would have which of the following fragments in its mass spectrum? m/z I 29 II 43 III 57 A)I B)II C)III D)All of the choices are correct.

Accepted Answer

Heptane, with the molecular formula of C7H16, will undergo fragmentation in the mass spectrometer. Fragment I at m/z 29 corresponds to the methyl cation (CH3+), fragment II at m/z 43 corresponds to the ethyl cation (C2H5+), and fragment III at m/z 57 corresponds to the t-butyl cation ((CH3)3C+). All three fragments are commonly observed in the mass spectrum of heptane, so the correct answer is D (all of the choices are correct).

Question 6

Pentane,C₅H₁₂,would have which of the following fragments in its mass spectrum? m/z II 29 III 43 IV 57 A)I B)II C)III D)All of the choices are correct.

Accepted Answer

Pentane has a molecular formula of C5H12, which means it can potentially form ions with the following m/z values: 
I) 15 (M+)
II) 29 (M-CH3)
III) 43 (M-2CH3)
IV) 57 (M-3CH3)

Therefore, all of the given choices (I-IV) are possible fragments in the mass spectrum of pentane.

Question 7

You observe a compound that exhibits a mass spectrum with peak at 160 and a peak at 162,both of equal intensity.This compound contains&#10;A)two chlorine atoms.&#10;B)one iodine atom.&#10;C)one bromine atom.&#10;D)two bromine atoms.

Accepted Answer

The compound must have two isotopes with a difference of two mass units, which indicates the presence of a chlorine and a bromine atom or a bromine and an iodine atom. However, option A and B can be ruled out since chlorine isotopes would give a peak at 158 and 160, and iodine isotopes would give a peak at 127 and 129. Therefore, the only option remaining is C, which corresponds to a compound containing one bromine atom (79Br) and one chlorine atom (35Cl), giving rise to peaks at 160 and 162 in the mass spectrum.

Question 8

In a typical mass spectrum,a smaller signal is observed at a mass 1 amu higher than the molecular ion peak.Why? A)Due to small impurities in the sample B)Machine error C)Because a small percentage of the compound will have a carbon that is the isotope ¹³C instead of ¹²C D)Because of fragmentation

Accepted Answer

The small signal observed at a mass 1 amu higher than the molecular ion peak is due to the presence of a small percentage of the compound that has a carbon atom with the isotope ¹³C instead of the more common isotope ¹²C. This is known as a isotopic peak and is a result of the natural abundance of isotopes in a sample.

Question 9

2,3-dimethylbutane gives a base fragment peak at m/z = 43.Which of the following is the most probable structure of the fragment ion?  &#10;A)I&#10;B)II&#10;C)III&#10;D)IV

Accepted Answer

The base peak at m/z = 43 is due to the loss of a methyl group from the tertiary carbon of the parent molecule (2,3-dimethylbutane). This can occur in structures III and IV, which both have a tertiary carbon with two methyl groups attached. However, structure IV has a secondary carbon adjacent to the tertiary carbon, which could result in a secondary fragmentation peak at m/z = 57. Therefore, the most probable structure of the fragment ion is III, which has only tertiary carbons in the vicinity of the methyl group loss.

Question 10

In electron impact mass spectrometry (EIMS),what is being detected?&#10;A)The molecular mass of the compound&#10;B)The molecular formula of the compound&#10;C)The mass to charge ratio of any ionic species&#10;D)The mass to charge ratio of any neutral species

Accepted Answer

In electron impact mass spectrometry (EIMS), the detector measures the mass to charge ratio (m/z) of ionized species. The technique does not directly detect neutral species, nor does it provide the molecular formula or the molecular mass of the compound without further analysis and interpretation of the mass spectrum.

Question 11

A compound X has a molecular ion peak in its mass spectrum at m/z 136.What information does this tell us about X? A)X has a molecular mass of 136. B)The molecular formula for X is C₈H₈O₂. C)The empirical formula for X is C₄H₄O. D)Both A (X has a molecular mass of 136)and B (The molecular formula for X is C₈H₈O₂)are true. E)Statements (X has a molecular mass of 136),(The molecular formula for X is C₈H₈O₂),and (The empirical formula for X is C₄H₄O)are all true.

Accepted Answer

The answer of A compound X has a molecular ion...

Question 12

What type of signal(s)would you observe in the mass and (or)infrared spectrum of the following compound?  &#10;A)A signal at 95 amu&#10;B)A signal at 94 amu&#10;C)Two signals at 95 and 94 amu&#10;D)A signal at 106 amu

Accepted Answer

The answer of What type of signal(s)would you observe in...

Question 13

What would be the molecular formula of rose oxide which contains C,H,and O and has two degrees of unsaturation and a molecular ion in its mass spectrum at m/z =154? A)C₁₀H₁₈O B)C₉H₁₄O₂ C)C₁₀H₂O₂ D)C₈H₁₀O₃

Accepted Answer

The answer of What would be the molecular formula of...

Question 14

What is the molecular ion,m/z,for the following molecule?  &#10;A)72&#10;B)70&#10;C)68&#10;D)66

Accepted Answer

The answer of What is the molecular ion,m/z,for the following...

Question 15

What is the molecular ion,m/z,for the following molecule?  &#10;A)72&#10;B)70&#10;C)68&#10;D)66

Accepted Answer

The answer of What is the molecular ion,m/z,for the following...

Question 16

Butane,C₄H₁₀,would have which of the following fragments in its mass spectrum? m/z I 15 II 29 III 57 A)I B)II C)III D)I and II

Accepted Answer

The answer of Butane,C₄H₁₀,would have which of the following fragments...

Question 17

Which of the following alkanes gives a significant fragment peak at m/z = 57?  &#10;A)I&#10;B)II&#10;C)III&#10;D)IV

Accepted Answer

The answer of Which of the following alkanes gives a...

Question 18

Hexane,C₆H₁₄,would have which of the following fragments in its mass spectrum? m/z I 29 II 43 III 57 A)I B)II C)III D)All of the choices are correct.

Accepted Answer

The answer of Hexane,C₆H₁₄,would have which of the following fragments...

Question 19

What type of feature will be observed in the EI mass spectrum of chlorobenzene? A)An absorption at 3100 cm^-1 B)A signal at 113 C)A signal at 112 and a signal at 114 in a ratio of 3:1 D)A signal at 112 and a signal at 114 in a ratio of 1:1

Accepted Answer

The answer of What type of feature will be observed...

Deck 30: Spectroscopy A: Mass Spectrometry