When we talk about amounts of substances, especially in chemistry we usually talk about the numbers of atoms of a particular type of element in a compound. Down at the atomic level it is not so much convenient to talk about grams and kilograms as it is to talk about percentages. So generally when we're looking at the proportionality of a particular element in a compound, we will look at its percentage mass. As we already know the relative atomic mass is an "average" of the atomic masses of the different isotopes of an element and the relative formula mass is something that we will naturally progress to now.


Put quite simply, the "relative formula mass" or "relative molecular mass" is simply the total of the relative atomic masses of the elements making up the compound. Let us illustrate this by a simple example:


Water Is a substance made up of two gases, hydrogen and oxygen in the ratio of 2 to 1, that is there are two hydrogen atoms for every oxygen atom giving is a chemical formula of H2O.



From an atomic point of view, we have three atoms in total and so proportionally water is 66% hydrogen and 33% oxygen by numbers of atoms (actually 66.6% recurring and 33.3% recurring but let's not worry about that just yet). What we need to evaluate now is how much hydrogen and oxygen is contained in water from a mass perspective. This is where our relative atomic masses  become useful. For the sake of simplicity, using whole numbers only we know that the relative atomic mass of hydrogen is one and that of oxygen is sixteen. The relative molecular mass or relative formula mass is therefore 1+1+16 which equals eighteen. We can therefore say that the relative formula mass of water is eighteen.


Q. Calculate the relative formula mass of iron (II) oxide


A. Iron (II) Oxide has the chemical formula of FeO, the element iron has a relative atomic mass of fifty-six (using integer values only) and the element oxygen has a relative atomic mass of sixteen (using, integer values again). The relative formula mass is therefore simply 56+16 = 72


That was quite an easy one because in iron (II) oxide the ratios of atoms are simply one-to-one, that is one iron atom bonds with one oxygen atom to form one molecule of iron oxide. Things start to get a bit more complicated  when we look at molecules with multiple numbers of elements, a natural progression from this would be to look at iron (III) oxide which has the chemical formula of Fe2O3


To calculate the percentage of each element in this compound we need to bring in the generic formula:



To illustrate this, let's use iron (III) oxide:


Ar Fe = 56


Ar O = 16


Mr = (56+56)+(16+16+16) = 160


We now have a relative formula mass for the compound of 160, to calculate the percentage mass of iron we multiply the relative atomic mass of iron by two, as we have two atoms of it and then divide this answer by the relative formula mass of the compound itself. So in this particular case, the percentage of iron is (112รท160) x 100 = 70%


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