Custom Search

 Dalton's Law of Partial Pressure

Dalton’s Law states that "The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone."

Ptotal = P1 + P2 + . . . Pn

• Pt is the total pressure of a sample which contains a mixture of gases
• P1, P2, P3, etc. are the partial pressures (in the same units) of the gases in the mixture

However, there is an unavoidable problem. The gas saturates with water vapor and now the total pressure inside the bottle is the sum of two pressures - the gas itself and the added water vapor.

WE DO NOT WANT THE WATER VAPOR PRESSURE.

So we get rid of it by subtraction.

Pdry gas = Ptotal - Pwater vapor

This means we must get the water vapor pressure from somewhere.

We get it from a table because the water vapor pressure depends only on the temperature, NOT how big the container is or the pressure of the other gas.

 Temp (oC) Vapor Pressure (mmHg) Temp (oC) Vapor Pressure (mmHg) -10 2.15 40 55.3 0 4.58 60 149.4 5 6.54 80 355.1 10 9.21 95 634 11 9.84 96 658 12 10.52 97 682 13 11.23 98 707 14 11.99 99 733 15 12.79 100 760 20 17.54 101 788 25 23.76 110 1074.6 30 31.8 120 1489 37 47.07 200 11659

Regents Questions

Example 1   A sample of hydrogen gas is collected over water at 14.0 oC.  The pressure of the resultant mixture is 113.0 kPa.  What is the pressure that is exerted by the dry hydrogen alone?

 Pdry gas=113.0 kPa(760mmHg/101.3 kPa)=847.8mmHg Pwater vapor= From Table=11.99mmHg Ptotal=? Pdry gas = Ptotal - Pwater vapor Pdry gas = 847.8mmHg - 11.99mmHg Pdry gas =835.8mmHg

Example 2 A mixture of oxygen, hydrogen and nitrogen gases exerts a total pressure of 278 kPa.  If the partial pressures of the oxygen and the hydrogen are 112 kPa and 101 kPa respectively, what would be the partial pressure exerted by the nitrogen.

 PO2=112 kPa PN2= ? PH2=101 kPa  Ptotal=278 kPa Ptotal = PO2 + PN2 + PH2 278 kPa = 112 kPa + 101 kPa + Pnitrogen Pnitrogen = 278 kPa - (112 kPa + 101 kPa) Pnitrogen = 65 kPa
 Questions Highlight to Reveal Answers 3. A mixture of neon and argon gases exerts a total pressure of 2.39 atm.   The partial pressure of the neon alone is 1.84 atm, what is the partial pressure of the argon? Ptotal = PNe + PAr  2.39atm=1.4 atm +PAr  PAr =0.55atm 4. A 450 cm3 sample of hydrogen is collect over water at 12oC.   The pressure of the hydrogen and water vapor mixture is 78.5 kPa.  What is the partial pressure of the dry hydrogen gas? PH2=78.5kPa(760 mmHg/101.3kPa)=589mmHg Ptotal = PH2 + PH2O Ptotal = 589mmHg + 10.52mmHg =  600.mmHg 5. 888 cm3 of oxygen are collected over water with a temperature of 25 oC.  The total pressure of the gases is 55.8 kPa.  What is the partial pressure of the dry gas? Ptotal =55.8kPa(760 mmHg/101.3kPa)=419mmHg Ptotal = PH2 + PH2O 419mmHg= PO2 + 23.76mmHg PO2 = 395mmHg