DALTON’S LAW OF PARTIAL PRESSURE
This law state that in a mixture of gases which do not react chemically together, the total pressure exerted by the mixture of gases is equal to the sum of the partial pressure of the individual gases that make up the mixture.
Mathematically, the law can be expressed as:
Ptotal = PA + PB +PC........Pn
Where Ptotal is the total pressure of the mixture and PA, PB, PC are the partial pressure exerted separately by the individual gases A, B, C that make up the mixture.
The pressure each constituent gas exerts is called partial pressure and is expressed as
Partial pressure of gas A (PA) = Number of moles of gas A x Ptotal
Total number of moles of gas in mixture
That is, PA = nA x Ptotal
nA + nB + nC
If the gas is collected over water, it is likely to be saturated with water vapour and the total pressure becomes
Ptotal = Pgas + Pwater vapour
Pgas = Ptotal – Pwater vapour
CALCULATION ON THE LAW
A gaseous mixture containing 64g of O2 and 70g of N2 exerts a total pressure of 1.8oatm. What is the partial pressure exerted by oxygen in the mixture?
Solution:
Molar mass of O2 = 16 x 2 = 32gmol-1
Molar mass of N2 = 14 x 2 = 28gmol-1
Number of mole of O2 = 64g = 2.0mole
32gmol-1
Number of mole of O2 = 70g = 2.5mole
28gmol-1
Total number of moles of gases in mixture = 2.0 + 2.5 = 4.5 mole
Partial pressure of O2 = 2.0 x 1.80 = 0.80atm
THEORY QUESTIONS
1. State Dalton’s of partial pressure.
2. Calculate the pressure at 27oC of 16.0g O2 gas occupying 2.50dm3
3. A certain mass of hydrogen gas collected over water at 10oC and 760mmHg pressure has a volume of 37cm3. Calculate the volume when it is dry at s.t.p (Saturated vapour pressure of water at 10oC =1.2mmHg)
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