2. Dalton’s Law of Partial Pressures

3. All gases are equal Suppose a flask contains 1.0 mole of Cl 2 which has a pressure of 1.0 atm. Another flask contains 1.0 mole of H 2 has a pressure of 1.0 atm. If the 1.0 mole of H 2 is added to the flask containing Cl 2, the total pressure will be 1.0 atm + 1.0 atm = 2.0 atm In general, pp gasA + pp gasB + … = P tot where pp = partial pressure

4. All gases are equal We can also work backwards from the total pressure to find the partial pressure of each gas if we know what percent or fraction each gas is compared to the total moles of gas.. Suppose that a volume contains equal amounts of two gases (50% of each) H 2 and O 2. The total pressure is 1.0 atm the pp H2 will be pp H2 = (0.50)1.0 atm = 0.50 atm.

5. All gases are equal We can also work backwards from the total pressure to find the partial pressure of each gas if we know what percent or fraction each gas is compared to the total moles of gas.. Suppose that a volume contains equal amounts of two gases (50% of each) H 2 and O 2. The total pressure is 1.0 atm the pp O2 will be pp O2 = (0.50)1.0 atm = 0.50 atm.

6. All gases are equal Suppose a sample of gas contains 0.5 moles of Br 2, 0.2 moles of He and 0.3 moles of Ar. What would the partial pressure of each gas be if the total pressure was 2.0 atm? a) the total moles are (0.5+0.2+0.3)= 1.0 moles b) the percent Br 2 will be (0.5/1.0) = 0.50 (50%) c) the percent He will be (0.3/1.0) = 0.30 (30%) d) the percent Ar will be (0.20/1.0) = 0.20 (20%) so, the partial pressure of Br 2 = 2.0*0.50 = 1.0 atm the partial pressure of He = 2.0*0.30 = 0.60 atm the partial pressure of Ar = 2.0*0.20 = 0.40 atm

7. This percent of moles of each gas is called the mole fraction. In general then, pp gasA = P total * mole fraction All gases are equal

8. Ex. What is the partial pressure of each of the gases when 0.34 moles of Ne, 0.24 moles of He and 0.56 moles of Rn are mixed to create a total pressure of 1.5 atm? Total moles = 0.34 + 0.24 + 0.56 = 1.14 moles pp Neon = 1.5 atm * (0.34/1.14) = 0.447 atm Likewise, pp He = 0.315 atm and pp Rn = 0.737 atm

9. Drying gases When gases are collected by the downward displacement of water, they contain some water vapour. The total pressure is then the pressure of the gas collected and the water vapour. P tot = pp gas + pp water In our lab today, you must remember to dry the gas. Partial pressure tables of water are found in appendix E of your textbook. We will use PV = nRT and solve for n, the total moles Then, using the partial pressure of butane, work out the moles of just butane in the gas collected. This will be used in the molar mass calculation.