What is Mole

A mole is the amount of a substance that contains the same number of particles as there are atoms in 12 grams of carbon 12 containing 6.0 x 10^23 atoms of carbon 12, therefore a mole represents 6x10^23 particles. This number is known as Avogadro's Constant (Na)

Calculating Relative Molar Mass (RMM)

RMM is calculated using the relative atomic mass (RAM) of the atoms in the molecules. The RAM values of atoms are the mass number of atoms and in a calculation the RAM values are given (the symbol for RAM can also be Ar). The The RMM is equal to the sum of the RAM values of all the atoms involved, Note that RAM & RMM are unitless.

NH3 find RMM

RAM: N=14 & H=1

RMM of NH3 = (1x14) + (3x1)

= 14 + 3 = 17

RMM of NH3 = 17

Molar Mass

Molar mass is the mass in grams of 1 mole of a chemical substance. The molar mass is given the unit of grams per mole

g/mol - gmol^-1

CO2, find molar mass

RAM; C=12 & 0=16

M of CO2 = (1x12) + (2x16)

12 + 32

44 gmol^-1

Mass Conversion

The molar mass can be used to calculate/convert a given mass of an element/compound to number or moles/a given number of moles to mass.

Number of Mole = given mass/molar mass

Units:

• Mole = mole(mol)
• Mass = grams
• Molar Mass = grams/mole (g/mol)

The Mole & Number of Particles

The number of particles in 1mol is 6x10^23, this can be used to convert a given number of particles in a substance to number of moles or vice versa.

mole=number of particles/6x10^23

The type of particles in a substance depend on the nature of the substance:

1. If it is an element for example a metal/a noble gas, the particles are individual atoms.
2. If it is a molecule such as a diatomic molecule such as Nitrogen N2 or just a covalent compound for example Carbon Dioxide CO2, the particles are molecules.
3. If it is an ionic compound for eg. Potassium Chloride KCL, the particles are formula units (made of ions).

Example: How many moles are there in 1.8 x 10^23 molecules of Nitrogen.

moles=1.8x1.^23 molecules/6x10^23 molecules/mole

= 0.3mol

The mole & Volume of gases

Avogadro's law states that equal volumes of all gases under the same conditions of temperature and pressure contains the same number of molecules. Since the number of molecules in each gas is 6 x 10^23 it follows that one mole at the same temperature and pressure occupies the same volume.

Molar Volume (Vm)

This is the volume occupied by 1 mole of gas. It depends on temperature and pressure:

1. At standard temperature and pressure (stp) where temperature is 0 degrees Celsius (Kelvin, K = degrees Celsius + 273 & degrees Celsius = Kelvin, K - 273) or 273K and pressure is 101.3 KPa (kilopascal) or 1 atmosphere, the molar volume is 22.4 dm^3 or 22400 cm^3.
2. At room temperature and pressure (rtp) where temperature is 25 degrees Celsius or 298K and pressure is 101.3KPa or 1 atmosphere, the molar volume of a ... 24 dm^3 or 2400cm^3

Molar volume can convert a given volume of a gas to moles or vice versa.

number of moles = give volume/molar volume (stp or rtp).

mole = Volume / Vm

Examples

1. How many moles are there in 27grams of water? RAM - Hydrogen=1, Oxygen=16

mole=mass/m

mole = ?, mass=27grams, m=?

H2O→m=(2x1) + (1x16) = 18g/mol

mole=27g/18g/mol = 1.5mol

2. What is the mass of 0.25mol of iron(III) sulfate?

RAM-iron=56, sulfur=32 & oxygen=16

m=(2x56) + (3x32) + (12x16)

= 400g/mol

mass=mole x m

mass= 0.25 x 400g/mol

= 100g

3. How many Ammonia molecules are there in 4.25g of Ammonia.

RAM - Nitrogen = 14 & hydrogen = 1

NH3

m = (1x14) + (3x1) = 17 g/mol

mole = mass/m = 4.25g/17gmol^-1

= 0.25

mole = molecules of NH3/6x10^23

molecules of NH3 = mole x (6x10^23)

= 0.25mol x 6 x 10^23

= 1.5 x 10^23 molecules

4. How many moles are there in 3.36 dm^3 of carbon dioxide at stp?

volume = 3.36dm^3

stp: Vm = 22.4 dm^3/mol

mole = ?

mole = volume/Vm

mole = 3.36dm^3 / 22.4dm^3/mole

= 0.15mol

5. What is the volume of 17.6g of Carbon Dioxide at 273K and 1 atmosphere?

RAM - Carbon=12 & Oxygen = 16

CO2

m = (1x12) + (2 x 16) = 44 g/mol

moles = mass/m →17.6g/44gmol^-1 = 0.4mol

mole = volume/Vm → volume = moles x Vm

= 0.4mol x 22.4 dm^3/mol

= 8.96dm^3