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A Partial Pressure and Stoichiometry Problems
Dr. Richard C. Sobers Jr.
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Partial Pressure Problem
A Sample of zinc metal reacts completely with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
The hydrogen gas is collected over water at 25oC. The gas volume is found to be 7.80L and the atmospheric pressure is 0.980atm. Calculate the mass of zinc metal that reacted.
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Partial Pressure Problem
A Sample of zinc metal reacts completely with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Moles H2 → Moles Zn
Moles Zn → Mass Zn
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Partial Pressure Problem
A Sample of zinc metal reacts completely with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
298K7.80L
How do we get moles of H2?
PV=nRT 0.980atm
But first look at the experiment
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Partial Pressure Problem
Gas is collected over water so that the atmosphere is not present. Water levels made equal so pressure inside is equal to that outside
But is the pressure inside due to hydrogen gas only?
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Vapor Pressure Curve for Water
At 25oC: PH2O = 23.8mmHg
Or PH2O = 0.0313atm
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Partial Pressure Problem
A Sample of zinc metal reacts completely with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
298K7.80L
PH2 = 0.949atm
PH2 = 0.980atm - 0.313atm
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Partial Pressure Problem
A Sample of zinc metal reacts completely with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
T = 298K V = 7.80LPH2V=nH2RT
PH2 = 0.949atm
(0.949)(7.80)=n(0.08206)(298)
R = 0.08206 L•atmK•mol
nH2 = 0.303moles
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Partial Pressure Problem
A Sample of zinc metal reacts completely with hydrochloric acid:
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
0.303mol
0.303 mol H21 mol Zn1 mol H2
⎛⎝⎜
⎞⎠⎟
65.39g Zn1 mol Zn
⎛⎝⎜
⎞⎠⎟ =19.8g Zn
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Stoichiometry with Gases
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Moles and Volumes of Gas
Quantity (# moles) of gas A
Volume (# L)
of gas A
Use PV=nRT
Or Use Molar Volume of Gas
At STP, 1 mol of a gas has a volume of 22.4L
If you know the molar volume at other pressure and temperature conditions then you can use that.
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Stoichiometry Problem With 2 Gases
For example: The combustion of propane !
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)
What volume of carbon dioxide is produced at STP if 0.500L of propane at 50.0psi and 25.0oC is combusted?
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Combustion of Propane
Quantity (# moles)
C3H8
Quantity (# moles)
CO2
Volume (# L) CO2
Volume (# L) C3H8
Use mole ratio
PV=nRT or Molar Volume
PV=nRT or Molar Volume
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)
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Combustion of Propane
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)25.0oC0.500L50.0psi
Volume → moles of propane: PV=nRT
Tprop = 298K Vprop = 0.500L Pprop = 3.40atm
nprop = PV/RT = 0.0695mol
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Combustion of Propane
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)Moles Propane → Moles Carbon dioxide
nprop = 0.0695mol
0.0695 mol C3H83 mol CO2
1 mol C3H8
⎛⎝⎜
⎞⎠⎟
= 0.209 mol CO2
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Combustion of Propane
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)Volume → moles of carbon dioxide:
nCO2 = 0.209molT = 273KP = 1.00atm
Could use PV = nRT:
STP
T = 273K
P = 1.00atmn = 0.209mol
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Combustion of Propane
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)Volume → moles of carbon dioxide:
Could also use the molar volume at STP (22.4L/mol):
0.209 mol CO222.4 L CO2
1 mol CO2
⎛⎝⎜
⎞⎠⎟
= 4.68 L CO2
nCO2 = 0.209mol
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Volume Ratios of Gases
The stoichiometric ratio applies to gas volumes if the gases are at the same temperature and pressure. This is the law of Guy-Lusaac.
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)
What volume of carbon dioxide gas at STP is produced if 0.500L (measured at STP) of propane are combusted?
0.500 L C3H83 L CO2
1 L C3H8
⎛⎝⎜
⎞⎠⎟
= 1.50 L CO2
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Volume Ratios of Gases
Quantity (# moles) of gas A
Quantity (# moles) of gas B
Volume (# L)
of gas B
Volume (# L)
of gas B
Stoichiometric Ratio
PV=nRT or Molar Volume
PV=nRT or Molar Volume
(both at same T and P)
Stoichiometric Ratio
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Volume Ratios of Gases
Back to this problem: The combustion of propane !
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)
What volume of carbon dioxide is produced at STP if 0.500L of propane at 50.0psi and 25.0oC is combusted?
Can we use volume ratios instead of mole ratios?
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Volume Ratios of Gases
Back to this problem: The combustion of propane !
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)
Can we use volume ratios instead of mole ratios?
298K
0.500L3.40atm
273K
V=?1atm
Yes - first calculate volume of propane if at STP!
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Volume Ratios of Gases
T1 = 298K
V1 = 0.500L
P1 = 3.40atm T2 = 273K
V2 = ??
P2 = 1atm
Volume (298K, 3.40atm) → Volume (STP) of propane:
P1V1
n1T1
= P2V2
n2T2
n1 = n2
For propane:
(3.40atm)(0.500L)298K
= (1atm)V2
273K
V2 = 1.56L
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Volume Ratios of GasesVolume propane (STP) → Volume CO2 (STP):
C3H8(g) + 10O2(g) → 3CO2(g) + 4H2O(g)
1.55LSTP
V=??STP
1.56 L C3H83 L CO2
1 L C3H8
⎛⎝⎜
⎞⎠⎟
= 4.68 L CO2