4.2 The Mole Concept
1 Define the mole in terms of a specific number of particles called Avogadro’s constant.
Mole is a quantity used to express the amount of substance as there are in the number of carbon atoms in 12 grams of Carbon-12.
Let’s say we have the element Calcium. It’s Nucleon Number (Protons + Neutrons) is 40. So 40g of Calcium will contain the same number of atoms as 12g of Carbon-12. We call this 1 mole of Calcium atoms.
1 mole of atoms has a certain quantity. It’s something like 6.02 x 1023, atoms, ions, molecules, whatever is in the substance. That’s like 602 000 000 000 000 000 000 000… I don’t have to have aced GCSE Mathematics to know that that’s one big number. Luckily, you
won’t have to do calculations with this ridiculously large number.
To sum this up, 1 mole of magnesium (24g) has 6.02 x 1023 magnesium atoms. 1 mole of calcium (40g) also has 6.02 x 1023 Calcium
atoms. This is the same for any element. The nucleon number of a substance is equivalent to one mole and has 6.02 x1023 atoms.
2 Use the molar gas volume, taken as 24 dm3 at room temperature and pressure.
If you get 4 flasks, with different elements, with standard conditions applied, and you weigh the four flasks with 4 distinct elements, you see something magical. Each flask has exactly 1 mole of each gas!
We call this Avagadro’s Law.
The conditions to get this magical effect (1 mole of each gas) are:
- Same Volume
- Same temperature
- Same pressure
At room temperature (298K) and 1atm pressure, the volume is 24 dm3.
3 Calculate stoichiometric reacting masses and reacting volumes of solutions; solution concentrations will be expressed in mol/dm3.
I’m going to split this into parts. Part 1 is calculating the volume of a gas:
This is simply a mathematical application of Avagadro’s Law.
Example Question: What is the volume occupied in 0.5 moles of a gas? (Standard Conditions Apply)
We know that 1 mole of a substance occupies 24dm3.
Therefore, 0.5 mole will occupy 0.5 x 24 dm3 = 12 dm3.
Example Question: Find the volume occupied by 36 g of Water in Standard Conditions?
Mr of H2O = (1+1) + 16 = 18
1 mole = 18 g
36 g will therefore be (36/18) = 2 moles
1 mole = 24dm3, as we learned just now, so as 36g of water is 2 moles, 2 moles = 24dm3 x 2 = 48 dm3.
We can also calculate the gas volume from an equation. To do this, we can simply find the ratio of coefficients subscripts and use the ratio to make the appropriate calculations. This may sound kind of fuzzy right now, but I’ll do an example to illustrate this:
- What volume of oxygen will react with 48 dm3 of hydrogen under standard conditions?
To approach this question, we should first write an equation expressing this reaction.
2H2 + O2 –> 2H2O
Now, there are three substances here. But we don’t need to make use of all of them! Let’s look carefully in the question. We only need to do calculations with the substance “oxygen” and “hydrogen”.
The coefficient subscript is the number in front of the substance.
For the reaction above:
2H2 + 1O2 –> 2H2O
The underlined values are the coefficient subscripts of the substance. Now the ratio is simply expressed as:
2: 1 : 2
Basically, what this this is saying is that 2 moles of hydrogen will react with 1 mole of oxygen to produce 2 moles of water!
Since 1 mole = 24 dm3, 48 dm3 = 24 dm3 * x ; x= 2
So 48 dm3 = 2 moles, that’s just 1st grade math.
Since the ratio of hydrogen and oxygen is 2:1, two moles of 2 will give you 4 moles of hydrogen.
And if you want to express the moles in dm3, as 1 mole = 24dm3, 4 moles = 24 x 4 = 96 dm3
If you get the concept of comparing coefficients and doing some simple multiplication problems, you are on your way to acing any problem given to you in the exam.
Let’s do one more
- The reaction between nitrogen and oxygen forms nitrogen oxide. What volume of nitrogen oxide is formed when 2 g of Nitrogen is allowed to burn?
When we solve this problem, let’s first ask ourselves, what is it that we want to find?
We want to find the volume of nitrogen oxide formed when 2g of nitrogen is burned.
Let’s first write down the reaction between nitrogen and oxide?
N2 + O2 → 2NO
Now, let’s write down the coefficients:
N2 = 1
O2 = 1
NO = 2
Ratios between coefficients:
We want to find the volume of nitrogen oxide formed.
We know that the Nucleon Number of Nitrogen = 14 g
2g of nitrogen = (2/14) mole = (1/7) mole
We know that 1 mole of nitrogen will react with 2 moles of nitrogen oxide
Since the mole reaction between nitrogen and nitrogen oxide is 1:1, (1/7) mole of nitrogen will react with (1/7)*2 = 2/7 mole of nitrogen oxide.
2/7 mole = 0.286 moles, and as 1 mole = 24 dm3,
Volume = 0.286 moles x 24 dm3 = 6.864 dm3
The concentration of a substance is the amount of solute dissolved in 1 dm3 of solution.
The formula to finding concentration is:
- Concentration = [Amount of constituent (mol) / Volume of the solution (dm3)]
We measure concentration using the units : mol/dm3
To find the amount of constituent or solute in a solution, we can rearrange the equation to get:
- Amount of constituent (mol) = Concentration (mol/dm3) x Volume (dm3)
To convert the moles to grams, simply use the traditional formula:
:Mass of Solute= Number of Moles x Molecular Mass