**Molar Mass**

- The mass of one mole of atoms is its relative atomic mass in grams, and is called a molar mass.
- A Molar mass is the mass of a substance containing the Avogadro Constant of particles.

Element | Relative atomic mass | Mass of 1 mol of particle | Molar Mass |

Hydrogen | 1 | 1 g | 1 g mol ^{-1} |

Carbon | 12 | 12 g | 12 g mol ^{-1} |

Oxygen | 16 | 16 g | 16 g mol ^{-1} |

Copper | 64 | 27 g | 27 g mol ^{-1} |

Iron | 56 | 56 g | 56 g mol ^{-1} |

**Example**

- The relative atomic mass of copper is 64, therefore the mass of 1 mole copper is
__64g__ - The RAM of sodium is 23, therefore the mass of 2 mole sodium is
__46g__ - The RAM of nitrogen is 14, therefore the mass of 2.5 mole l nitrogen is
__35g__ - The RAM of calcium is 20, therefore the mass of 0.3 mole calcium is
__6g__

**Conclusion **

##### Note:

The relationship between the mass of a substance and the number of mole of the particles in the substance can be summarised by using the following equation:

n= m Molar Mass

If you are given the mass of substance and asked to find the number of mole of the substance (or vice versa), the problem can be solved by using this equation.

**Example**:

Find the number of mol of atoms in 4.6g sodium [Relative atomic mass: Na=23]

**Answer**:

Number of mole, [n = frac{{4.6}}{{23}} = 0.2mol]

**Example**

How many moles of each substance are there in 191 g NaOH [Relative atomic mass: Na=23, O=16, H=1]

**Answer**:

Relative Formula Mass of NaOH = 23 + 16 + 1 = 40

Number of mole of 191g NaOH,

[n = frac{{191}}{{40}} = 4.775mol]

__Question__What is the mass of 7.12 mol NaI. [Relative atomic gas: Iodine = 131; Sodium = 23]

**Answer**:

The relative formula mass of NaI = 23 + 131 = 154.

The mass of 7.12 mol NaI

= Number of mole x Relative Foemula Mass of NaI

= 7.12 x 154

= 1096.48g

## Mass of Substance and Number of Particles

- Sometime, you may be given the mass of a substance, and asked to find the number of particles (or vice versa).
- To solve the problem, we must find the number of mole of the substance then only we can find the number of particles (or mass of the substance).

**Example**:

How many molecules are there in 16 g of oxygen (O

_{2})? [ Ar: O =.16, Avogadro Number = 6 x 10

^{23}]

**Answer**:

The relative molecular mass of O

_{2}= 2(16) = 32

Number of mole of oxygen molecules = Mass of oxygen Molar mass of oxygen = 16 g 32 g mol -1 =0.5 mol

__Question:__Find the number of atoms in 34g of ammonia gas (NH

_{3}). [Relative atomic mass: Nitrogen: 14; Hydrogen: 1]

**Answer**:

Relative molecular mass of NH

_{3}

= 14 + 3(1)

= 17

Number of mole of ammonia = 34 17 =2 mole

Number of NH

_{3}molecules = 2 x 6.02 x 10

^{23}= 1.204 x 10

^{24}

Each NH

_{3}molecule contain 4 atoms (1 nitrogen atom and 3 hydrogen atoms).

Therefore, number of atoms

= 4 x 1.204 x 10

^{24}

= 4.816 x 10

^{24}

Question 2

Find the mass of methane (CH_{4}) that contain 1.806 x 10^{23} of methane molecules. [Relative atomic mass: Carbon: 12; Hydrogen: 1]

**Answer**:

The relative molecular mass of CH_{4}

= 12 + 4(1)

= 16

Number of mole of C H 4 = 1.806× 10 23 6.02× 10 23 =0.3 mole

Mass of CH_{4}

= Number of mole x molar mass

= 0.3 x 16 = 4.8g