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ALevel

Chemistry-9701

Relative Masses of Atoms and Molecules

Paper-3

Relative Masses of Atoms and Molecules
 

1: What Is The Unified Atomic Mass Unit?

> The unified atomic mass unit as one-twelfth of the mass of a carbon-12 atom.

2: Define Relative Atomic Mass, Ar, Relative Isotopic Mass, Relative Molecular Mass, Mr, And Relative Formula Mass In Terms Of The Unified Atomic Mass Unit.


1: Relative Atomic Mass

>The ratio that is deduced from the average mass of one atom of an element to one-twelfth of the mass of an atom of carbon-12.

Note: The need to use the average mass of an atom of a particular element is because most elements are mixtures of isotopes.

> For example, the exact Ar of hydrogen is known to be 1.0079. This is very close to the whole number value of “1” and most periodic tables give the Ar of hydrogen as 1.0 but some elements present in the Periodic Table have values that are not whole numbers.

>For an illustration, the Ar for chlorine is 35.5. This is due to chlorine-containing two isotopes in a sample of chlorine. Now, if we take a sample of chlorine “chlorine-35” makes up about three-quarters of the chlorine atoms and “chlorine-37” makes up about a quarter.

2: Relative Isotopic Mass

>The ratio of the average mass of one isotope of an element to one twelfth of the mass of an atom of carbon 12.

>For example, the relative isotopic mass of carbon-13 is known as 13.00. If we are aware of both the natural abundance of every isotope present of an element and the isotopic masses they have, we can calculate the relative atomic mass of the element. To explore the relevant data we use an instrument called a Mass Spectrometer.

3: Relative Molecular Mass

> The ratio of the average mass of one molecule of an element to one-twelfth of the mass of an atom of carbon 12.

>We can find the relative molecular mass by adding up the relative atomic masses of all the atoms that are present inside a molecule:
 
                     
   Cambridge International Examinations Lawrie Ryan, Roger Norris 

4: Relative Formula Mass

> The ratio of the average mass of one formula unit of an element to one-twelfth of the mass of an atom of carbon 12.

Note: A formula unit is an empirical formula used for an ionic compound. It is the lowest possible ratio of the cations (positive ions) and Anions (negative ions) of a given ionic compound.

>For example, Table Salt (sodium chloride) has a formula unit of NaCl, meaning that the sodium and chloride ions are in a ratio of 1:1.

> In short: for compounds that have ions we use the term relative formula mass. It is calculated in a very similar way to the relative molecular mass. It is also given the same symbol, Mr. For example, for magnesium hydroxide:
        
   Cambridge International Examinations Lawrie Ryan, Roger Norris 

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