Amount of Substance

What is the amount of substance?

Fig. 1 - Oxygen is a diatomic particle. It exists in pairs so the elementary entity for oxygen is O2

What is the mole?

A chemical mole is another way of saying an exact quantity. Just like we say 'a dozen' to mean 'twelve things', a mole is '602 hexillion things'. That's 602,200,000,000,000,000,000,000! We write it as for short.

Fig. 2 - A mole is 602 hexillion things!

The number is also known as Avogadro's constant (L). It is named after an Italian scientist called Amedeo Avogadro. He discovered that equal volumes of gases contain the same number of molecules when under the same conditions. Scientists used this discovery to calculate Avogadro's constant.

Why is the mole important?

The mole is essential when we talk about chemical reactions. Have a look at this equation:

$$2H_2O+O_2\rightarrow 2H_2O$$

We can say "oxygen and hydrogen react to make water"

or

"two molecules of hydrogen and one molecule of oxygen react to make water".

Did you notice that we need two times as many molecules of hydrogen as we need oxygen? So if we had 1 mole of hydrogen and 1 mole of oxygen they would have the same number of molecules - Avogadro's constant. To make sure we have twice as many hydrogen molecules, we need twice as many moles of hydrogen.

That means we can also say "2 moles of hydrogen react with 1 mole of oxygen to make 2 moles of water".

The mole is helpful because it allows you to read chemical equations by the number of moles of each substance. You can then figure out the exact amounts of substances that are reacting.

What is relative mass?

Scientists measure the mass of an atom by comparing it to \(\frac {1} {2} \) the mass of a neutral carbon-12 atom. We call this relative mass.

Why do we use relative masses?

We use relative masses because atoms are so tiny that using their actual weights makes calculations tricky. Instead of using the actual mass of atoms in problems, scientists compare all atoms to a standard atom: carbon-12. They use carbon-12 because it is a stable isotope and they can measure its mass accurately. Carbon-12 is the isotope of carbon with 6 protons and 6 neutrons. They found out the carbon-12 atom weighs \(6\times 10^{-23}\space grams\) .

Fig. 3 - Carbon-12 Olive

Scientists gave carbon-12 a mass of 12u because they found it easier to say "carbon-12 weighs 12u". By comparing the weight of all other atoms to carbon-12 they discovered the hydrogen atom weighs \(\frac {1} {12} \) of the carbon-12 atom. So they gave hydrogen a relative mass of 1u.

hydrogen = carbon mass ÷ 12

=12u ÷ 12

=1u

What is relative atomic mass?

The atomic mass of an element will vary from one isotope to another. The figure we see for an element's atomic mass on the periodic table is the relative atomic mass.

What is relative molecular mass?

What is percentage yield?

When we compare the actual amount of product we get from a chemical reaction to the amount we theoretically could have got, it is called Percentage Yield.

What is theoretical yield?

Theoretical yield is the yield you would get if all the reactants in your experiment turned into a product.

Why is actual yield lower than theoretical yield?

Actual yield is often lower than theoretical yield because:

1. Some of the reactants don't convert to a product.

2. Some of the reactants get lost in the air (if it's a gas).

3. Impurities stop the reaction.

4. Unwanted by-products get produced in side-reactions.

5. The reaction reaches equilibrium.

The ideal gas equation

Gases have three natural properties: volume, pressure, and temperature. Scientists have known for a long time that there is a relationship between these three properties. The Ideal Gas Law is an equation that explains the relationship between the natural properties of gases. Here’s how you write the ideal gas equation:

$$ PV\space = nRT $$

P: pressure (Pa)

V: volume \(m^3\)

n: number of moles

R: The gas constant

T: temperature (K)

Before the ideal gas law, scientists had noticed other relationships between the temperature, pressure, and volume of gases.

• Scientists found that if the amount of gas and its pressure stay the same, when you change the temperature the volume also changes. Raising thetemperature increases the volume. Lowering the temperature decreases thevolume. Scientists named this discovery Charles’s Law.

• They also realised that volume and pressure are inversely related. If volume goes up the pressure goes down. The opposite is true too! This relationshipis called Boyle's Law.

What is an ideal gas?

That means the gas molecules have no volume and no attractive forces between each other. When you think about it, that can't be true at all! So there is no such thing as an ideal gas.

Fortunately, most real gases behave in an ideal way.

What is the kinetic theory of gases?

This theory is based on a few vital assumptions.

• Gases have molecules that move randomly in straight lines.

• The molecules of a gas behave like rigid spheres.

• When the molecules of gas collide with the sides of a container it causespressure.

• When the molecules collide with each other and against the container they donot lose kinetic energy. (The collisions are completely elastic.)

• The temperature of the gas relates to the average kinetic energy of its molecules.

• The molecules have negligible intermolecular forces between them.

• The volume occupied by the molecules is negligible and relative to the volume of the container.