Chemistry - Relative Mass

Problem Set


How do we express the mass of an atom? It is nearly 0kg0kg! By using the Mass Spectrometer, scientists can now determine the mass of atoms. For example, Oxygen atom is approximately 2.657×1026kg2.657\times10^{-26}kg. However, do you have any idea how heavy or light 2.657×1026kg2.657\times10^{-26}kg is? So we can see that it is not easy to write or memorize if we use the exact mass of the atom.

So, scientists compare the mass of each atom. They use the mass of 112Carbon-12\frac{1}{12} \text{Carbon-12} (12C)(^{12}C) as a standard and compare it with other atoms. For example,

Mass of 112Carbon-12:Mass of Oxygen atom\text{Mass of }\frac{1}{12} \text{Carbon-12}:\text{Mass of Oxygen atom}

1:161:16

Then, the value 1616 is the Relative Atomic Mass of Oxygen atom.

We write it as Ar(O)=16A_r(O)=16.

What about the mass of a molecule? We use the same idea, but called Relative Molecular Mass. What we have to do is sum up all the relative atomic mass of each atoms in the molecule. For example,

Mass of 112Carbon-12:Mass of water, H2O\text{Mass of }\frac{1}{12} \text{Carbon-12}:\text{Mass of water, }H_2O

1:2×1+161:2\times1+16

1:181:18

Then, the value 1818 is the Relative Molecular Mass of water H2OH_2O molecule.

We write it as Mr(H2O)=18M_r(H_2O)=18.

Note that the difference are:

  1. Relative Atomic Mass is for atom. C,H,O,N,PC, H, O, N, P

  2. Relative Molecular Mass is for molecule. CO2,H2O,NH3CO_2, H_2O, NH_3

Some other notes:

  1. At the beginning, scientist did not use the mass of 112\frac{1}{12} Carbon-12\text{Carbon-12} as the standard but Hydrogen\text{Hydrogen} instead.

  2. Relative Mass has no unit as it is a comparison between mass of atoms or molecules.

Note by Christopher Boo
5 years, 9 months ago

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Comments

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Why is hydrogen not used now?

shuvayan ghosh dastidar - 5 years, 9 months ago

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Yes, I will go through this history later.

Christopher Boo - 5 years, 9 months ago

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Bcos carbon combines with most compounds while hydrogen doesn't,that's why it becomes easy to calculate relative mass using C

Akhil Bansal - 4 years, 4 months ago

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Hi, Christopher I am new to chemistry and I hope that someone will reply to this comment How do you know that the value 16 is the Relative Atomic Mass of Oxygen atom? Had you already known that it is 16 or you calculated it right while you post this?

Daniel Lim - 5 years, 9 months ago

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An element is identified by the number of protons that its nucleus contains. Oxygen atom has 8 protons by definition. Similarly, Carbon is identified with 6 protons in its nucleus.

Now, for both of these atoms, in their standard isotope, the protons are accompanied by the same number of neutrons. i.e, Oxygen has 8 Protons and 8 Neutrons, while Carbon has 6 Protons, and 6 Neutrons.

The mass of a proton and a neutron is approximately same. Thus, if we define :

1A.M.U1 A.M.U (Atomic Mass Unit) == Mass of a Proton, then, clearly,

Mass of Oxygen atom (standard isotope) =8+8=16A.M.U= 8+8 = 16 A.M.U, and,
Mass of Carbon atom (standard isotope) =6+6=12A.M.U= 6+6 = 12 A.M.U

Now,
If we take 112\frac{1}{12}th of the mass of carbon atom =112×12=1A.M.U= \frac{1}{12}\times 12 = 1 A.M.U, then this is defined as the standard Atomic Mass.

Thus, clearly,
Relative Mass of Oxygen atom =16= 16

Anish Puthuraya - 5 years, 9 months ago

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Thanks Anish Puthuraya for further explanation as I did not include any information about protons and neutrons.

Daniel-

If you don't know the number of protons or neutrons of an atom, you can always refer to the Periodic Table Scroll down to 9.6 until you saw a table full of elements, the value below each elements is their relative atomic mass.

For relative molecular mass, we don't have a table but it is easy too sum up all the relative atomic mass of atoms present in that molecule. For example, NH3NH_3 has 1 NN and 3 HH is 14+3×1=1714+3\times1=17.

Hope this helps :)

Christopher Boo - 5 years, 9 months ago

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Thanks to both of you

I had already known the atomic number before this

Daniel Lim - 5 years, 9 months ago

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Is it only for C,H,O,N,PC, H, O, N, P and CO2,H2O,NH3CO_2, H_2O, NH_3?

Daniel Lim - 5 years, 9 months ago

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No, we use the term atomic mass for atoms. There are 115115 elements as of now. So, there are 115115 atoms. And they all of atomic masses. In short, the atomic mass of an atom tells you how much massive that atom is compared to 112\frac{1}{12} of the mass of a C12C-12 atom.

We use the term molecular mass for molecules. There are a lot of different molecules in the world, so they all have their own molecular masses [not just CO2,H2OCO_2, H_2O and NH3NH_3. They were just examples].

Mursalin Habib - 5 years, 9 months ago

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Thank you

Daniel Lim - 5 years, 9 months ago

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@Daniel Lim Always glad to help!

Mursalin Habib - 5 years, 9 months ago

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Instructive post.

Soham Dibyachintan - 5 years, 9 months ago

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GOOD THING

sachin verma - 5 years, 9 months ago

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great post

fairooze waziha - 5 years, 9 months ago

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Very instructive post.

Vivian Sudhir - 5 years, 9 months ago

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