Back to: CHEMISTRY SS1

**Welcome to class! **

In today’s class, we will be talking more about gas laws – Avogadro’s law. Enjoy the class!

**GAS LAWS II – AVOGADRO’S LAW**

This law states that equal volume of all gases at the same temperature and pressure contain the same number of molecules. This means that 1 mole of any gas at s.t.p has a volume of 22.4dm^{3}.

**GAY LUSSAC’S LAW OF COMBINING VOLUMES**

It states that when gases react they do so in volumes which are simple ratios to one another and the volumes of the products if gaseous, provided that the temperature and pressure remain constant.

**CALCULATION ON THE LAW**

Calculate the volume of oxygen required to burn 500cm^{3} of methane completely.

**Solution:**

The equation for the reaction is:

2CH_{4(g)} + 3O_{2(g)} → 2CO_{2(g)} + 2H_{2}O_{(g)}

By Gay Lussac’s law,

2 volumes of CH_{4} requires 3 volumes of O_{2} for complete combustion

Therefore, 2cm^{3} of CH_{4} requires 3cm^{3} of O_{2}

500cm^{3} of CH_{4} will require Xcm^{3} of O_{2}

Xcm^{3} of O_{2} = 500cm^{3} x 3cm^{3} ) / 2cm^{3} = 750cm^{3
}

Thus, 750cm^{3} of O_{2} is required.

**EVALUATION**

- State the Gay Lussac’s law of combining volumes
- 40cm
^{3}of hydrogen was sparked with 160cm^{3}of oxygen at 100^{o}C and 1atm. Determine the volume of oxygen left after the reaction.

**GRAHAM’S LAW OF DIFFUSION**

It states that the rate of diffusion of a gas is inversely proportional to the square root of its density at constant temperature and pressure.

Mathematically,

R α 1/√d

R = k/√d where k is a constant

Comparing the rate of diffusion of two gases:

__R _{1}__ =

__√d__R

_{2 }_{2}√d

_{1}

In terms of relative molecular mass, M

R α 1/√M

For two gases,

__R _{1}__ =

__√M__R

_{2 }_{2}√M

_{1}

But the rate of diffusion is reciprocal of time, R =1/t

That is,

__R _{1}__ =

__t__R

_{2 }_{2}t

_{1}

From the inverse relationship, we can deduce that the less dense a gas is, the higher the rate of diffusion and vice versa.

**CALCULATION**

- A given volume of SO
_{2}diffuses in 60 seconds. How long will it take the same volume of CH_{4}to diffuse under the same condition (SO_{2}= 64, CH_{4}= 16)

**Solution:**

Using the expression:

__t _{1}__=

__√M__t

_{2 }_{2}√M

_{1}

t_{2} = __√M _{2 }__x t

_{1}=

__√16 x__60seconds = 30seconds

√M1 √64

**GENERAL EVALUATION/REVISION**

- State Graham’s law of diffusion
- Under the same condition of temperature and pressure, hydrogen diffuses 8 times as fast as gas Y. Calculate the relative molecular mass of Y.
- State the following rule/principle: (a) Hund’s rule of maximum multiplicity (b) Aufbau principle
- Write the electronic configuration of (a) oxide ion, (b) Aluminium ion, (c) potassium (d) phosphorus.

**READING ASSIGNMENT**

New School Chemistry for Senior Secondary School by O.Y. Ababio, Pg 86-92

**WEEKEND ASSIGNMENT**

- 400cm
^{3}of a gas X diffuses through a porous pot in 2 minutes. Calculate the rate at which X diffuses. A. 6.3cm^{3}s^{-1}20cm^{3}s^{-1 }C. 200cm^{3}s^{-1}D. 3.33cm^{3}s^{-1}# - The relationship between the density (d) of gas and the rate at which the gas diffuses is A. R = kd
- R= k/√d C. R = k√d D. k/d*
- Calculate the minimum volume of oxygen required for the complete combustion of a mixture of 20cm
^{3}CO and 20cm^{3}of H_{2}. A. 10cm^{3}20cm^{3}C. 40cm^{3}D. 60cm^{3} - If sulphur (iv) oxide and methane (CH
_{4}) are released at the same time at opposite ends of a tube, the rate of diffusion will be in the ratio A. 2:1 B. 4:1 C. 1:4 D. 1:2 - ‘Equal volume of all gases at the same temperature and pressure contain the same number of molecules’ is a state of which law A. Avogadro’s law B. Boyle’s law Charles’ law D. Chemical law

**THEORY**

- Arrange the following gases in order of increasing rate of diffusion: CO, SO
_{2}, H_{2}S, NO_{2}and O_{2}. - The vapour densities of O
_{2}and Cl_{2}are 16 and 36 respectively. If 60cm^{3}of O_{2}diffuses through a porous partition in 14 seconds, how long will it take 1000cm^{3}of Cl_{2}to diffuse through the same partition?

In our next class, we will be talking about **Identification and Types of Alloys**. We hope you enjoyed the class.

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