1.3 MASSES OF ATOMS AND COMPOUNDS & AMOUNT OF SUBSTANCE

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Welcome again If you don't know me yet, my name is Aneta. In today's post, and in the next three posts, I am going to try and tell you everything you need to know about chemical calculations. For some of you this might be difficult and you will want to just through your book out of the window (please don't do it!), and for some of you it will be easy. It also depends on how you feel about maths. I personally love maths, so I really enjoyed this topic. Either way, I am sure the next couple of posts will solve all your issues with maths in chemistry. Also, remember to visit my youtube channel where you can practice you skills even more by completing past paper questions.

The first thing you might want to know, is that there are couple of definitions that just need to stick in your brain. Let's start with them:

Relative atomic mass (Ar) - average mass of one atom relative one-twelfth the mass of one atom of carbon-12.

Relative isotopic mass - average mass of one atom of an isotope relative to one-twlfth the mass of one carbon-12.

Isotope - atom with the same number of protons but different number of neutrons.

Now, you probably think what does that 'relative to one-twelfth the mass of one atom of carbon-12' mean? All, this means is that when we calculate mass of an atoms, we always compare it to the atom of carbon-12. You always look at the relative atomic mass.

An example? Silicon is 28 times heavier than hydrogen ( Atomic mass of hydrogen is 1 and that of silicon is 28).

Concluding, we consider the atomic number as the mass of an atom. And our last definition:

Relative molecular mass (relative formula mass), Mr - average mass of a molecule relative to one-twelfth the mass of one carbon-12.

This time we also compare the mass to carbon-12, however, Mr is different to Ar because it is the mass of the whole molecule. You basically add all of the Ar in the molecule.

There are no units when we calculate the mass of an atom or a molecule.

Let's try some examples of calculating relative molecular mass (he first part of the image below):

1. In order to complete these questions you need a periodic table to be able to check atomic mass of atoms. So, the atomic mass of carbon is 12 and there is only one carbon. The atomic mass of oxygen is 16 and we have two oxygens therefore we multiply it by 2. Add everything together and you're done. This is one of the easiest compounds to find the Mr for.

2. This one has a bracket and everything that is in the bracket has to be multiplied by the little number outside the bracket. Again you add up all the Ar in this compound, remembering to multiply the molecule in the bracket by 3.

3. This example is the most difficult that there is going to be. The first part, which is sodium carbonate you calculate as usual. Then, the dot shows you that the second part of molecule is water. Big number at the front of water means there are ten water molecules. So, you find the Mr of water and then multiply it by 10. At the end you add all of the Ar that you had and you get the Mr of the whole molecule.

There are many things you can do when you know the Mr of a compound. And an example of this is that you can find % composition of different atoms in a compound. (The second part of the image)

e.g. they ask us to find the percentage composition of copper in the following compound (all this means is the percentage of copper in the compound OR how much copper there is compared to the whole compound). This is not very hard to do once you know how to find Mr.

The first thing you do is find the Mr of a whole compound and then the Ar of copper. Divide the relative atomic mass of copper over the Mr of the whole compound and multily by a 100. This is your answer.

The next type of questions that you can be asked in the exam is using ratios:

This unit will mostly be just examples because this is the best way to explain chemical calculations.

1. What I always do in this tyype of question is write what I already know. So, 400g produced 600cm cubed That's why I wrote 400=600 and then you are asked to find how much 2.25g would give. So, again I write that down and it equals to '?' because we need to find this. The next part is to do a 'cross multiplication' This means you go across and multiply ( 2.25x 600) and then divide by 400.

2. This is exaclty the same in the question. You are given that 0.623g produced 162cm cubed. Then, what is the volume if 1.00g were used. So, 0.623=162 and 1.00='?'. Cross multiplication: 1x162 and divide by 0.623. The technique is always the same but the numbers change. The context of the question may ask you about anything but you can always use cross multiplication for this type of question.

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Amount of substance

When they ask you to find the amount of a substance, what they mean is actually how many moles there are. And here is another definition for you:

One mole - it is the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12.

Again this is all compared to carbon-12. In 12g of carbon-12 there are 6.02x10^23 particles and this is a standard number for one mole. This number is called the Avogardo's constant.

e.g. if you have 8.3mol of a subtance and 1 mol equals to 6.02x10^23 particles, then you need to multiply this standard by 8.3 to get the answer.

Furthermore, if one mole is related to carbon-12 and the relative atomic mass is also related to carbon-12. All these informations conclude to give that 1 mol of any atom is equal to the relative atomic mass of this atom. Also,1 mole of any compound is equal to the relative molecular mass of this compound.

The last thing for this post is an equation that combines all that we talked about today:

number of moles (mol)= mass(g)/molar mass

This is one of these equations you must be able to remember! You can also rearrange the equation depending on what the question is asking you to find.

Molar mass- is the mass of one mole of a substance. This is the Mr of a substance.

Examples!!!:

1. You have a water molecule here (calculate the Mr) and mass for it is also provided. You are asked to find the number of moles. So, no rearranging is needed. Put the numbers into the equation and put it into your calculator.

2. This time you are trying to find the Mr. So the steps are: rearrange the equation, put in the number of moles and mass and type it into your calculator. The second part of the question tells us to find the number of molecules. 1 mole is equal to the Avogadro's constant. Therefore, 0.004mol multiply by the Avogard's constant and you will get the number of molecules for this substance.

3. This one is a little bit more tricky because there are a lot of steps you need to take. But let's start slow and we will get there:

- Find the Mr of the compound;

- Find the number of moles of this compound; (we need the number of moles in order to find the number of atoms)

- 1 mol= Avogadro's constant .: 0.25mol= 1.5x10^23

- C3H8 has 11 atoms (8+3);

So, at the end you multiply 11 by 1.5x10^23.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!