Molecular ions

Yatayyat

14

How do I do this? I don’t have a clue on what to do??

Thanks.

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Reply 1

username3694440

15

im guessing C, since the others all have oxygen in them and C doesn't

Reply 2

Yatayyat

OP

14

Original post by hello2906

im guessing C, since the others all have oxygen in them and C doesn't

But how does that explain the answer?

Reply 3

LiiHyde

14

Original post by Yatayyat

But how does that explain the answer?

It doesn't

By molecular ion - you know in the M/Z graphs they give you where you look at molecular ion peaks? The mass is determined by the largest mass recorded as the ion formed is simply the compound only missing 1 electron

so
A = C₄H₈O Which has a molecular mass of 72gmol^-1
B = C₄H₇OH = 72gmol^-1
C = C₅H₁₂= 72gmol^-1
D = C₅H₁₀O = 86gmol^-1

Each molecule still has the same mass as it's molecular ion too
A simply forms C₄H₈O⁺ when an electron is knocked off

Reply 4

username3694440

15

Original post by Yatayyat

But how does that explain the answer?

is the answer C? Because all of the others are a collection of the same Cs and Hs, so will all have precisely the same molecular masses since theyre made up of exactly the same atoms. Oxygen has a slightly different mass to Cs/Hs, so the compound will have a slightly different Mr to the others.

Reply 5

Yatayyat

OP

14

Original post by LiiHyde

It doesn't

By molecular ion - you know in the M/Z graphs they give you where you look at molecular ion peaks? The mass is determined by the largest mass recorded as the ion formed is simply the compound only missing 1 electron

so
A = C₄H₈O Which has a molecular mass of 72gmol^-1
B = C₄H₇OH = 72gmol^-1
C = C₅H₁₂= 72gmol^-1
D = C₅H₁₀O = 86gmol^-1

Each molecule still has the same mass as it's molecular ion too
A simply forms C₄H₈O⁺ when an electron is knocked off

Wait how does D have a mr of 86 isn’t 72?

The answer is C, because it is the only one without oxygen. All of the others have the same combination of carbon, hydrogen and oxygen. The precise Mr, i.e. down to six decimal places will be different for different molecular formula, even if the integral value appears the same.

Reply 7

Yatayyat

OP

14

Original post by charco

The answer is C, because it is the only one without oxygen. All of the others have the same combination of carbon, hydrogen and oxygen. The precise Mr, i.e. down to six decimal places will be different for different molecular formula, even if the integral value appears the same.