Organic Chemistry 4.1 - ExpDetStruc - Empirical Fomula and Mass Spectrometry

Question 1

Elemental microanalysis is used to calculate the ___ formula of a compound, by determining the masses of which atoms in an organic compound?

Answer 1

empirical.
CHONS

Question 2

The masses of Carbon, Hydrogen, Oxygen, Nitrogen and Sulfur present in an ___ compound can be determined through ___ ___.

Answer 2

organic, elemental microanalysis

Question 3

What does an empirical formula show?

Answer 3

the simplest ratio of the elements in a molecule

Question 4

Elemental microanalysis can be determined from which two things?

Answer 4

1. masses of products of combustion.
2. percentage by mass of products

Question 5

Combustion product masses:
‘1.75 mg of an organic compound is combusted completely, producing 3.51 mg of CO2 and 1.43 mg of H2O.’

Calculate the empirical formula of the compound.

Answer 5

Step 1: find the mass of elements, in the products, that are guaranteed to be in the mystery compound (start with carbon and hydrogen). EG for carbon, multiply the mass of carbon dioxide (3.51mg) by 12/44 (carbon’s mass in carbon dioxide).
Step 2: Once you’ve done this for carbon and hydrogen, add the masses together. If it equals the mass of the mystery compound combusted, go to step 3. If not, subtract it from the mass of mystery compound, and take this as the mass of a third element - if CO2 and H2O were produced, its obv oxygen. - but it could be any of ‘CHONS’ so be ready for that
Step 3: Calculate moles of each element (n=m/GFM)
Step 4: Calculate mole ratio by dividing all by lowest mole value - if you don’t get (very close to) whole numbers, you may have made a mistake - or maybe try X2 or X3 all values

Question 6

Combustion product masses:
‘1.75 mg of an organic compound is combusted completely, producing 3.51 mg of CO2 and 1.43 mg of H2O.’

Outline the steps you should take to calculate the the empirical formula of the compound.

Answer 6

1. find mass of elements
2. calculate moles of elements (n=m/GFM)
3. calculate mole ratio (divide all by lowest mole value)

Question 7

Combustion product masses:
‘1.75 mg of an organic compound is combusted completely, producing 3.51 mg of CO2 and 1.43 mg of H2O.’

Calculate the molecular formula of the compound, given that is has a GFM of 88.

Answer 7

Add up the GFMs of the elements in the empirical formula, and divide the molecular formula GFM by that number. that tells you how many of the empirical formula fits into the molecular

Question 8

Percentage by Product Mass:
‘Calculate the empirical formula for a compound with 75% carbon and 25% hydrogen by mass.’

How do you work it out?

Answer 8

imagine you have 100g of the substance (you’re working with a ratio so it doesn’t matter at all).
divide each percentage by its GFM - so carbon would be 75/12, and that tells you moles (n=m/GFM).
divide each mole value by the lowest mole value, to find the simplest ratio

Question 9

Mass spectrometry can be used to accurately determine the ___ and ___ ___ of a compound.

Answer 9

GFM, structural fomula

Question 10

In mass spectrometry, a small sample of an ___ compound is ___ by ___ energy ___.

Answer 10

organic, bombarded, high, electrons

Question 11

What happens in mass spectrometry?

Answer 11

a small sample of an organic compound is bombarded with high energy electrons

Question 12

What does this this high energy electron bombardment do?

Answer 12

it removes electrons from the compound, creating positively charged molecular ions (called parent ions)

Question 13

What do these parent ions then do?

Answer 13

break apart into smaller, positively charged ‘fragments’

Question 14

This technique allows a mass spectrum to be obtained, showing a plot of the ___ ___ of the ions detected (on the y axis) , against the ___-___-___ ratio, on the x axis.

Answer 14

relative abundance, mass-to-charge ratio (m/z)

Question 15

Describe the 4 steps to achieving a mass spectrum.

Answer 15

1. organic compound bombarded with high energy electrons.
2. electrons removed, creating a parent ion
3. parent ion breaks apart, producing positive fragments
4. graph plotted, with relative abundance on y axis and mass to charge ratio (m/z) on x axis

Question 16

Parent ions usually split into one ___ and one ___ fragment.
However, only positive ions can be ___.

Answer 16

positive, neutral.
detected

Question 17

When looking at a mass spectrum, how do you determine the mass of the molecular/parent ion?

Answer 17

look at the molecular ion peak - which is the peak with the highest m/z value - IE the farthest right

Question 18

Organic compounds each produce a ___ mass spectrum, however there will be lots of similarities between them due to the peaks of the ___.

Answer 18

unique.
fragments

Question 19

The m/z value of a fragment is essentially the GFM of a ___, minus one ___.

Answer 19

hydrocarbon, hydrogen (ie one gram / ‘one GFM unit’)

Question 20

What is the m/z value of the following fragments? (no.5 is a parent ion - pentane)
1. CH3+
2. OH+
3. C2H5+
4. C3H7
5. C5H12+

Answer 20

m/z:
1. 15
2. 17
3. 29
4. 43
5. 72

Question 21

When an organic compound Is bombarded with high energy electrons, and becomes a parent ion, its GFM (does/does not) change.

Answer 21

does not (the mass of 1 electron is negligible)

Question 22

What does the farthest right line on a mass spectra represent?

Answer 22

the parent ion

Question 23

The most abundant fragment is known as the ___ ___, and is assigned a relative abundance value of ___.

Answer 23

base peak, 100%

Question 24

‘Why does fragment X have an m/z value that is so low?’
*fragment X’s m/z value does not match any fragments you’d expect

Answer 24

because it has a +2 charge