Dalton's law of partial pressure states that total pressure of the mixture of inert gases is equal to the sum of partial pressures of each gas present in the mixture.

1. DALTON’S LAW
OF PARTIAL
PRESSURE
XI FDC CHEMISTRY
CHAPTER 4
SIDRA JAVED

2. DALTON’S LAW OF PARTIAL
PRESSURE
The total pressure in a container is the sum
of the pressure each gas would exert if it
were alone in the container.
The total pressure is the sum of the partial
pressures.
PTotal = P1 + P2 + P3 + P4 + P5 ...
2
John Dalton

3. 3
Dalton’s Law

4. EXAMPLE OF DALTON’S LAW
Air is a mixture of non reacting
gases. 78% N2 , 20% O2, 0.93 % Ar
, 0.03% CO2 , traced of Ne, He, Kr,
H2, water vapors and pollutant
gases such as oxides of sulphur
and nitrogen are found in air.
The total pressure exerted by air
is equal to the sum of partial
pressure of each gas.
4

5. RELATIONSHIP BETWEEN PARTIAL PRESSURE
AND NO. OF MOLES OF GAS
Let nA, nB and nC be the no. of moles
of gas A, B and C respectively
Partial pressures of these gases are
PA, PB and PC
As we know that, PV= nRT
For Gas A, 𝑃𝐴 =
𝑛 𝐴 𝑅𝑇
𝑉
….(i)
For Gas B, 𝑃𝐵 =
𝑛 𝐵 𝑅𝑇
𝑉
….(ii)
And for Gas C, 𝑃𝐶 =
𝑛 𝐶 𝑅𝑇
𝑉
….(iii)
Total pressure, 𝑃𝑡 =
𝑛 𝑡 𝑅𝑇
𝑉
….(iv)
where nt = nA + nB + nC
Dividing equation (i) by (iv)
𝑃 𝐴
𝑃𝑡
=
𝑛 𝐴 𝑅𝑇
𝑉
÷
𝑛 𝑡 𝑅𝑇
𝑉
=
𝑛 𝐴 𝑅𝑇
𝑉
×
𝑉
𝑛 𝑡 𝑅𝑇
=
𝑛 𝐴
𝑛 𝑡
𝑃𝐴 =
𝑛 𝐴
𝑛 𝑡
× 𝑃𝑡
𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑓 𝑎 𝑔𝑎𝑠
=
𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑔𝑎𝑠
𝑡𝑜𝑡𝑎𝑙 𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠
× 𝑡𝑜𝑡𝑎𝑙 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
∴ 𝑃𝐴 ∝ 𝑛 𝐴
Similarly, 𝑃𝐵 ∝ 𝑛 𝐵 and 𝑃𝐶 ∝ 𝑛 𝐶
5

6. RELATIONSHIP BETWEEN PARTIAL PRESSURE
AND NO. OF MOLE FRACTION OF GAS
We know that:
𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑓 𝑎 𝑔𝑎𝑠 =
𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑔𝑎𝑠
𝑡𝑜𝑡𝑎𝑙 𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠
× 𝑡𝑜𝑡𝑎𝑙 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
𝑃𝑎𝑟𝑡𝑖𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑜𝑓 𝑔𝑎𝑠
𝑡𝑜𝑡𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒
=
𝑛𝑜. 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑔𝑎𝑠
𝑡𝑜𝑡𝑎𝑙 𝑛𝑜.𝑜𝑓 𝑚𝑜𝑙𝑒𝑠
𝑃 𝑖
𝑃𝑡
=
𝑛 𝑖
𝑛 𝑡
or 𝑃𝑖 =
𝑛 𝑖
𝑛 𝑡
× 𝑃𝑡
But,
𝑛 𝑖
𝑛 𝑡
= 𝑋𝑖 , where X is called mole fraction. So,
Pi = Xi Pt or Piα Xi
6

7. APPLICATIONS OF
DALTON’S LAW OF
PARTIAL
PRESSURE

8. COLLECTION OF GASES OVER
WATER
When a gas is collected over
water, provided that the gas is
insoluble in water, gas
becomes moist. The total
pressure of moist gas will be
equal to:
Pt = Pgas + Pwater
Or Pgas = Pt – Pwater
The pressure due to water
vapors is called aqueous
tension.

9. RESPIRATION AT HIGHER
ALTITUDESPartial pressure of O2 in
outside air (159 g/cm2) is
higher than partial pressure of
O2 in lungs (116g/cm2)
However at higher altitude, air
pressure decreases, hence
respiration becomes difficult.
Pilots may have un-
comfortable breathing in a
non-pressurized cabin where
the partial pressure of oxygen
is about 150 g/cm2

10. RESPIRATION BY DEEP SEA DIVERS
At the depth of sea, air pressure increases 5 times than normal
pressure.
Regular air cannot be used in a diver’s tank because partial pressure
O2 would be 795 g/cm2.
Deep sea divers use a mixture of 96% He and 4% O2 to breath.
SCUBA (self-contained breathing apparatus) contains compressed air
to breath. As the divers returns to surface, it becomes hazardous for
him if not handled properly.
As the divers comes up, the pressure of surrounding water drops.
Consequently the compressed air in lungs expands.
Surfacing is done very slowly so that the compressed air may escape
out from the lungs without damage to them.

11. HOW DEEP SEA DIVERS RESPIRE
ON RETURN FROM SEA?
Air contains 80% N2. if N2 is present in divers tank, the solubility
of N2 in blood can increase with increase in pressure in very
deep sea.
If the diver returns quickly and relatively at low pressure at
surface, he will face a life threatening condition called Bends.
The diver must use different mixture of gases such as He and
O2 or spend many hours decompression chamber after a dive.
The pressure in decompression chamber is slowly lowered over
many hours. N2 comes out of blood slowly and disposed off
safely.

12. END OF
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