Percentage Composition
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Percentage Composition

August 23, 2019


A company manufacturing washing soda had to
specify the percentage of water of crystallization on its bottle. So what is the percentage of
water of crystallization in this washing soda? Now we are given the atomic masses of each
of the elements. We have to find the percentage composition of the water of crystallization.
So this is the entire washing soda, we have to find the percentage composition of this
species. So how do we do that? We take the amount of this species by the total amount
and we multiply it by 100, to calculate the percentage composition.
So this is the percentage composition, that is, the total weight of the particular species,
for which we have to calculate the percentage composition, that present in one molecule
divided by the gram molecular weight of the compound and since we have to find the percentage
composition, we multiply it by 100. So now let’s calculate.
We are given the formula for washing soda, which is Na₂CO₃.10H₂O. This 10H₂O
is the water of crystallization which is present in the washing soda. So we are given that
the relative atomic mass of oxygen is 16, sodium is 23, carbon is 12 and for hydrogen
it’s 1. So let’s calculate first for the water of crystallization. We have H₂O. The
relative atomic mass of each hydrogen is 1, so we get 1×2 and relative atomic mass of
each oxygen is 16, so we get 2+16 and when we express the relative molecular mass in
grams, that is, known as the Gram Molecular Mass. So gram molecular mass is abbreviated
as G.M.M. So now we can express this in terms of grams, so we get 18 grams. But in washing
soda we have 10 molecules of water. So for one molecule 18 grams, so for 10 molecules
we get 10×18 which is equal to 180 grams. This is the total amount of water of crystallization,
present in the washing soda. Now let’s calculate the gram molecular mass of washing soda. Let’s
take this as 1. Let’s take this as 2. So for washing soda we have 2 atoms of sodium,
so each has the relative atomic mass 23 multiplied with 2, plus each carbon has the relative
atomic mass 1, since there is only 1 carbon atom, each oxygen has 16 and there are 3.
So let’s first calculate for this Na₂CO₃. So we get this is equal to 46+12+48, so we
get 16 and 10. And now when we add this to 10H₂O, that we have already calculated,
so we get 106+180 for water of crystallization (that we have already calculated) we get 286
grams. So we get the gram molecular mass of the entire water of crystallization equal
to 180 grams, the gram molecular mass of washing soda equal to 286 grams.
So the gram molecular weight or the gram molecular mass of 10H₂O (which is the entire amount
of water of crystallization, present in washing soda) its 180 grams. And that of washing soda
itself is 286 grams. So now to calculate the percentage composition,
we take the gram molecular weight of 10 molecules of water divided by gram molecular weight
of washing soda and we multiply it with 100. So we get, on calculating 62.9%. So the percentage
composition of the water of crystallization in washing soda is 62.9%. So this is the amount
or this is the value that should be present or written on the bottle of washing soda. Urea is the very important fertilizer for
soil. This is also a very important fertilizer for many crops. This is because it has nitrogen
and nitrogen is very good and healthy for plants. So now, what is the percentage composition
of nitrogen which is present in this fertilizer that is Urea? The formula of urea is {CO(NH₂)₂}
and we are given the relative atomic masses of each of the species. So for oxygen, nitrogen,
carbon and hydrogen, we are given the relative atomic masses. So now let’s calculate the
percentage composition of entire nitrogen present in this urea.
So this is urea. First, let’s calculate the amount of nitrogen which is present in
this urea. So if you will observe, since nothing is written here, this is 1. So we have 2 atoms
of nitrogen in urea. Each nitrogen has a relative atomic mass of 14. So for nitrogen which is
present, since we have 2 atoms we get 14×2 a.m.u. And the gram molecular mass of nitrogen
becomes 28 grams. So this is the entire amount of nitrogen which is present in urea. Now
let’s calculate the gram molecular mass of urea itself. So from the formula of urea,
we know we have 1 atom of carbon which is 12, we have 1 atom of oxygen…. Then we have
2 atoms of nitrogen
and we have 2×2….4 atoms of hydrogen. Each hydrogen has atomic mass of 1 and there are
4 hydrogen atoms. So now let’s calculate. We get 12+16+28+4. So this
is equal to 12+16, we get 28, plus 28 we get
56, plus 4 we get 60. So the gram molecular mass of the entire content of nitrogen, present
in urea is 28 grams. And the gram molecular mass of urea itself is 60 grams. Now we have
to calculate the percentage composition of nitrogen in urea. So what do we do?
We have both the gram atomic…..gram molecular masses. So we now divide the gram molecular
mass of nitrogen by the gram molecular mass of urea and to calculate the percentage composition
we multiply it with 100. So what do we get? On calculating we get 46.67%. So this is how
we get the percentage of the entire amount of nitrogen present in urea. So to calculate
the percentage composition of any species, we calculate the gram molecular mass of that
species in one molecule, we divide it by the gram molecular mass of the total compound
and we multiply it by 100.

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