Lesson 3, Topic 3
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Dalton’s Law of Partial Pressure

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John Dalton, in his experiment and study of gases, observed that each gas in the mixture exerted its own pressure on the walls of their container as if no other gas was present in the container.

In 1801, his observation was put forward as Dalton’s law of Partial Pressure which states that:

In a mixture of gases that do not react chemically together, the total pressure exerted by the mixture is equal to the sum of the partial pressures of the individual gases present in the mixture.

This implies that if gases a, b, c ….n are in a vessel, the total pressure, PT, exerted on the walls of the vessel can be determined as:

PT = PA + PB + PC = ………….Pn

Where PA, PB, PC …………….Pn are the partial pressure of individual gases a,b,c, ……n respectively.

Example

1. 200cm3 of nitrogen gas at a pressure of 500mmHg and 100cm3 of carbon (iv) oxide at a pressure of 50mmHg were introduced into a 150cm3 vessel. What is the total pressure in the vessel?

Solution

Volume of the vessel = 150cm3

$$\frac{Volume \; of \; the \; gas}{Volume \; of \; the \; vessel} \; \times \; \frac{Pressure \; of \; the \; gas}{1}$$

According to Daltons law of Partial Pressure

PTotal = Pa + Pb + Pc ………..+ Pn

PTotal = PN2 + PCO2

PTotal = 666.67mmHg  + 33.33mmHg

PTotal = 700mmHg

2. 50cm3 of a gas x and 30cm3 of a gas y occupies a container with a capacity of 80cm3. If the total pressure of the gas mixture is 760mmHg. Calculate the partial pressures of gases x and y respectively.

Solution

Volume of gas y = 50cm3

Volume of gas x = 30cm3

Total Volume of the container = 80cm3

Total Pressure of the gas = 760mmHg

Partial Pressure of gas x = $$\frac{50}{80} = \frac{760}{1}$$

Partial Pressure of gas x = 475mmHg

Partial Pressure of gas y = $$\frac{30}{80} \; \times \; \frac{760}{1}$$

Partial Pressure of gas y = 285mmHg

PTotal = PX + PY

i.e 475 + 285  = 760mmHg.

3. Calculate the pressure of the dry gas collected over water at 60C and 765mmHg. Given that the vapour pressure of water at 60C is 7mmHg.

Solution:

At 60C, pressure of the dry gas

= Pressure of wet gas – vapour pressure of water

765 – 7 = 758mmHg

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