Well, here goes, let's see what we can all make of this inscrutable, challenging and vital topic.
The topic of the Mole starts a bit further back with the attempt to determine atomic mass and molecular mass.
Incredibly difficult tasks need inventive ways to solve them.
Solving the problem of measuring atomic mass taxed the minds of chemists throughout the 19th century.
In principle the way into the problem came with the dawning of the idea of comparative mass or relative mass.
In other words, how do you solve the problem of measuring atomic mass?
Answer: you don't, instead you compare atoms against one another to see which is the more massive.
Take a standard atom and determine that its mass is the standard mass which we'll denote as 1 and see how many times heavier other atoms are compared to this particular atom.
If it happens that you choose the standard mass as the mass of the lightest atom then it works.
And for some years in the late 19th century and early 20th century that's what chemists did.
They used hydrogen (but you'd guessed that already I think) as the standard mass.
Chemists developed intricate machines to measure the relative mass of other elements relative to hydrogen's mass.
Couple the measurement of relative atomic mass with Mendeleev's brilliant piece of imaginative thinking that gave us the Periodic Table and in the late 19th century you had a rough set up.
Adding precision was the real challenge.
It was left to a Brit, Aston by name who around 1920 invented the first mass spectrometer to measure relative atomic mass.
His design remains the basic design students are asked to study on most A level and college courses today.
It's like this:
The process of determining the relative mass works like this:
Vaporisation: Mass spectrometers these days are used to determine the fragmentation pattern of the breakdown of an organic molecule in the machine.
So molecules are usually injected in the machine as liquids which are then vaporised.
Ionisation: Atoms or molecules are then bombarded by fast moving electrons which ionise the atoms or break up (fragment) and ionise the organic molecules (M).
So M + e-(fast) = M+ + 2e- (slow)
(Other particles/fragments M- and M• also form together with M+)
Acceleration: As these ionised fragments (M+, M- and M•) enter an electric field, the positively charged fragments only are accelerated into a magnetic field.
All other fragments (M- and M•) are evacuated from the machine.
Deflection: Positively charged fragments (M+) enter a magnetic field and are deflected according to their mass the lighter particles more than the heavier ones.
Detection: A detector picks up the incredibly small changes in charge as a small current at different mass values ( the machine is calibrated with a substance of known mass ) and converts these into a fragmentation pattern print out.
Here is a mnemonic to help remember this detail VIADD
Vaporisation
Ionisation
Acceleration
Deflection
Detection
Use it if it helps.
The standard atomic mass is no longer hydrogen but the isotope of Carbon: Carbon -12.
Carbon-12 is assigned a Relative Isotopic Mass of 12.00000 amu (atomic mass units)
Relative Atomic Mass is the weighted isotopic average mass for an element relative to 1/12th the mass of the Carbon-12 isotope.
So let's use the Chlorine mass spectrum (that's for atomic chlorine: Cl, not molecular chlorine Cl2) to work out its RAM:
Here is the print out showing the two isotopes of atomic chlorine:
Look at the peaks: Chlorine-35 is three times higher than chlorine-37.
Chlorine-35 is three times more abundant on earth than chlorine-37
Therefore, 25% of chlorine atoms are mass 37 and 75% are of mass 35.
Therefore, the RAM of chlorine is
And this number, 35.5, is what you'll find on your common copy of the Periodic Table.
You'll find more precise calculations on the web if you go looking, like this one below:
Just to finish you'll probably have realised by now that Relative Isotopic Mass refers to the relative mass of one particular isotope of an element relative to the Carbon-12 isotope.
Relative Formula Mass is the relative mass of a compound's formula relative to the mass of the Carbon-12 isotope.
You just have to add up the RAM's of the right number of elements to calculate an RFM.
Like NaCl or KH2PO4 or (NH4)2SO4
You should satisfy for yourself that you an calculate these values for these RAM's correctly and then try: (NH4)2Fe(SO4)2.6H2O which is the formula of Mohr's Salt the hexagonal crystals shown below:
Similarly with Relative Molecular Mass (Mr)when the compound is an actual discrete group of atoms like H2SO4, just add up the relative atomic masses of the right number of element atoms.
You can check the answers to these problems from the web quite easily.
Next we'll look at how you can use RAM data in calculations involving formulae and equations.
Here is a typical problem on this topic:
You can get answers to these questions if you add a comment and email contact
The pages of the Mole topic are listed below.
Click on any one to go there (When they are written and uploaded!!!)
Mole (2) Amount of substance and Molar Mass
The Mole (3) Using the mole to determine the simplest (empirical) formulae
The Mole (4) Using the mole to determine equation stoichiometry
Mole (5)
Mole (6)
The topic of the Mole starts a bit further back with the attempt to determine atomic mass and molecular mass.
Incredibly difficult tasks need inventive ways to solve them.
Solving the problem of measuring atomic mass taxed the minds of chemists throughout the 19th century.
In principle the way into the problem came with the dawning of the idea of comparative mass or relative mass.
In other words, how do you solve the problem of measuring atomic mass?
Answer: you don't, instead you compare atoms against one another to see which is the more massive.
Take a standard atom and determine that its mass is the standard mass which we'll denote as 1 and see how many times heavier other atoms are compared to this particular atom.
If it happens that you choose the standard mass as the mass of the lightest atom then it works.
And for some years in the late 19th century and early 20th century that's what chemists did.
They used hydrogen (but you'd guessed that already I think) as the standard mass.
Chemists developed intricate machines to measure the relative mass of other elements relative to hydrogen's mass.
Couple the measurement of relative atomic mass with Mendeleev's brilliant piece of imaginative thinking that gave us the Periodic Table and in the late 19th century you had a rough set up.
Adding precision was the real challenge.
It was left to a Brit, Aston by name who around 1920 invented the first mass spectrometer to measure relative atomic mass.
His design remains the basic design students are asked to study on most A level and college courses today.
It's like this:
The process of determining the relative mass works like this:
Vaporisation: Mass spectrometers these days are used to determine the fragmentation pattern of the breakdown of an organic molecule in the machine.
So molecules are usually injected in the machine as liquids which are then vaporised.
Ionisation: Atoms or molecules are then bombarded by fast moving electrons which ionise the atoms or break up (fragment) and ionise the organic molecules (M).
So M + e-(fast) = M+ + 2e- (slow)
(Other particles/fragments M- and M• also form together with M+)
Acceleration: As these ionised fragments (M+, M- and M•) enter an electric field, the positively charged fragments only are accelerated into a magnetic field.
All other fragments (M- and M•) are evacuated from the machine.
Deflection: Positively charged fragments (M+) enter a magnetic field and are deflected according to their mass the lighter particles more than the heavier ones.
Detection: A detector picks up the incredibly small changes in charge as a small current at different mass values ( the machine is calibrated with a substance of known mass ) and converts these into a fragmentation pattern print out.
Here is a mnemonic to help remember this detail VIADD
Vaporisation
Ionisation
Acceleration
Deflection
Detection
Use it if it helps.
The standard atomic mass is no longer hydrogen but the isotope of Carbon: Carbon -12.
Carbon-12 is assigned a Relative Isotopic Mass of 12.00000 amu (atomic mass units)
Relative Atomic Mass is the weighted isotopic average mass for an element relative to 1/12th the mass of the Carbon-12 isotope.
So let's use the Chlorine mass spectrum (that's for atomic chlorine: Cl, not molecular chlorine Cl2) to work out its RAM:
Here is the print out showing the two isotopes of atomic chlorine:
Chlorine-35 is three times more abundant on earth than chlorine-37
Therefore, 25% of chlorine atoms are mass 37 and 75% are of mass 35.
Therefore, the RAM of chlorine is
And this number, 35.5, is what you'll find on your common copy of the Periodic Table.
You'll find more precise calculations on the web if you go looking, like this one below:
Relative Formula Mass is the relative mass of a compound's formula relative to the mass of the Carbon-12 isotope.
You just have to add up the RAM's of the right number of elements to calculate an RFM.
Like NaCl or KH2PO4 or (NH4)2SO4
You should satisfy for yourself that you an calculate these values for these RAM's correctly and then try: (NH4)2Fe(SO4)2.6H2O which is the formula of Mohr's Salt the hexagonal crystals shown below:
Similarly with Relative Molecular Mass (Mr)when the compound is an actual discrete group of atoms like H2SO4, just add up the relative atomic masses of the right number of element atoms.
You can check the answers to these problems from the web quite easily.
Next we'll look at how you can use RAM data in calculations involving formulae and equations.
Here is a typical problem on this topic:
1The mass spectrum of the isotopes of element Germanium is shown in the diagram below
 Use data from the diagram to calculate the relative atomic mass of Germanium. Give your answer to one decimal place. ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ ............................................................................................................................................ Define the term relative atomic mass ............................................................................................................................................. |
Identify the ion responsible for the peak at 72 ............................................................................................................................................
Identify which one of the isotopes of Germanium is deflected the most in the magnetic field of a mass spectrometer. Give a reason for your answer.
Isotope ................................................................................................................................
Reason ...............................................................................................................................
In a mass spectrometer, the relative abundance of each isotope is proportional to the current generated by that isotope at the detector.
Explain how this current is generated.
............................................................................................................................................
............................................................................................................................................
............................................................................................................................................
............................................................................................................................................
Ge and Zn are different elements.
Explain why the chemical properties of 70Ge and 70Zn are different.
............................................................................................................................................
...........................................................................................................................................
Identify which one of the isotopes of Germanium is deflected the most in the magnetic field of a mass spectrometer. Give a reason for your answer.
Isotope ................................................................................................................................
Reason ...............................................................................................................................
In a mass spectrometer, the relative abundance of each isotope is proportional to the current generated by that isotope at the detector.
Explain how this current is generated.
............................................................................................................................................
............................................................................................................................................
............................................................................................................................................
............................................................................................................................................
Ge and Zn are different elements.
Explain why the chemical properties of 70Ge and 70Zn are different.
............................................................................................................................................
...........................................................................................................................................
You can get answers to these questions if you add a comment and email contact
The pages of the Mole topic are listed below.
Click on any one to go there (When they are written and uploaded!!!)
Mole (2) Amount of substance and Molar Mass
The Mole (3) Using the mole to determine the simplest (empirical) formulae
The Mole (4) Using the mole to determine equation stoichiometry
Mole (5)
Mole (6)
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