Q9. Here are several ways to work this problem: Method 1. Use PV = nRT to calculate the moles of each gas, hydrogen and nitrogen. n = PV / RT where P is the pressure in atmospheres, V is the volume in liters, R is the gas constant 0.08206 L atm mol�? K�?, and T is the Kelvin temperature. Call them nH2 and nN2. Then add them together to get the total moles of gas, ntot. Now you can plug back into PV = ntotRT again (but this time V is 0.090 L) and solve for the pressure. T is still 273.15 K (0°C). Method 2. Alternatively, you can add the two volumes, 0.042 L + 0.069 L, and plug the total volume into PV = nRT, where P is 1 atm and T is 273.15 K, and solve for n. You should get the same value of n this way as ntot above. Plug back into PV = nRT again and solve for the pressure as before. Method 3. Or, since ntot is not changing and T is also constant at 273.15 K, you can simply use P1V1 = P2V2 where P1 = 1 atm, V1 = 0.042 L + 0.069 L, P2 = ?, and V2 = 0.090 L, and solve for P2. This is the simplest way. 
Q11. For this one, you just have to plug into the combined gas law,
P1V1 = P2V2
T1 T2
and solve for V2. Be sure the units of pressure are the same on both sides (either both in atm or both in mmHg) and that the temperatures are both in Kelvins.
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