PARTIAL PRESSURES
A great deal of care was used in encoding all of the relationships for
partial pressures into the spreadsheet. You simply enter the data you
have into the appropriate window and hit the STEPPER (CALCULATE) button.
It will derive all the data that can be derived. For values that apply
to the entire container such as temperature, it is suggested that you
just enter them into the PP Sum row for simplicity. If you have a
data value that will not map onto the spreadsheet, then some external
calulcations may be required but this is a rare exception.
1.
If a gas contains 0.78 moles of N2, 0.23 moles of O2, and
0.22 moles of Ar, what are the pressures of these gases if the total
pressure is 1.6 atm?
Solution:
-We enter N2+O2+Ar% and hit the
SUBMIT button.
-We enter 0.78 in the moles column for N2, 0.23 in the moles
column for O2, and 0.22 in the moles column of Ar.
-The "Partial Gas?" column
should be set to Yes for all three gases.
-Next, we hit the GASES/VAPORS
button and enter 1.6 in the atm column of the "PP Sum" row.
-The "Ideal?"
column should be set to Y for all three gases.
-Finally, we hit the STEPPER (CALCULATE)
button and read the pressures for the 3 gases in the atm column: ( N2=1.015
,O2=0.2992 ,Ar=0.2862 ).
2.
O2 is at the top of a sealed container with liquid H2O in it. The
temperature inside is 24 degrees C and the volume of the gas is 0.128
liters. The pressure of the gas which is a combination of O2 and water vapor
is 762 mmHg. The pressure of water vapor at this temperature is 22.4 mmHg.
Calculate the mass in grams of the O2 contained.
Solution:
-We
enter O2+H2O% and hit the SUBMIT button.
-We enter 0.128 in the liters column
of "PP Sum".
-We hit the GASES/VAPORS button and enter 24 in the degC column
of "PP Sum".
-We indicate that O2 is ideal and H2O is not an ideal gas.
-We
enter 22.4 in the mmHg column of H2O.
-We enter 762 in the mmHg column of the
"PP Sum" row.
-We hit the STEPPER (CALCULATE) button and then hit the BASIC QUANTITIES
button.
-We read the grams for O2 (0.164).
3. Nitroglycerin decomposes according
to the equation:
C3H5(NO3)3 => CO2 + H2O + N2 + O2.
What volume of
gases are produced and collected at 1.3 atm, 26 degC if 2.4e2 grams of
nitroglycerin is exploded? What are their partial pressures? Assume all gases
are ideal.
Solution:
-We enter the unbalanced formula
with a % sign at the end to indicate partial pressure
and hit the SUBMIT button.
-We indicate "PP gas?" as "y" for CO2,H2O,N2, and O2.
-We enter 2.4e2 for
grams of nitroglcerin.
-Then we hit the gases button and indicate "Ideal" as
"y" for the four product gases.
-Finally we enter 26 in the degC column for
"PP Sum" and 1.3 atm in the atm column for "PP Sum" and hit the STEPPER (CALCULATE)
button.
-We read the answer for volume of gas in the liters column of "PP
Sum" (144.7).
-The partial pressures in atms are, going from left to right:
(0.538,0.448,0.269,0.0448).
4. As a use of Van der Waal's gas equation, consider CCl4 and O2 gas
in the same 3 liter container at a temperature of 300 degK. The pressure
is 2 atm inside. The partial pressure of CCl4 is 1.7 atm.
How many moles of CCl4 are inside?
Solution:
-As a first approximation, lets treat the gases as ideal.
-We enter CCl4+O2% and hit SUBMIT.
-Then we enter 3 in the liters column of
"PP Sum" and 300 in the degK column of "PP Sum".
-Finally, we enter
2 in the atm column of "PP Sum" and 1.7 in the atm column of CCl4.
-We check "Y" for "PP gas?" for both
gases and we indicate both gases are "Ideal".
-We hit the stepper
button and read the moles of CCl4 (0.207).
-Now let's repeat the above calculation with Van der Waal's gas equation.
-We enter CCl4+O2*% or CCl4+O2%* (the * means use Van der Waal's equation) and enter the
data exactly as before.
-Note the the "Ideal?" column has been removed
and now we have two columns in its place: "Waal a" and "Waal b". These
correspond to the two constants in his equation.
-For CCl4, a=20.4
atm*L^2/mol^2, b=0.138 L/mol.
-For O2, a=1.36, b=0.0318.
-We enter these
values and hit the STEPPER (CALCULATE) button.
-We read the moles of CCl4 (0.218).
Note there is a small difference between the two calculations.
5. Enter the following formula and hit SUBMIT.
Note * is placed at the end of the equation
to indicate the use of Van der Waal's gas equation.
Cr2O7[2-] + H[+] + C2O4[2-] => Cr[3+] + CO2 + H2O *%
If the CO2 and H2O are captured as gases in the same 3 liter container and
they have a combined pressure of 2 atm at a temperature of 25 degC,
how many grams of H2O is contained?
Solution:
-After entering the above formula with an *% at the end, we indicate the
gases that are "PP gas?" and we fill in the a and b constants for Van
der Waal's equation.
-For CO2, a=3.59, b=0.0427.
-For H2O, a=5.46, b=0.0305.
-We fill
in the rest of the data supplied and hit the STEPPER (CALCULATE) button. We read
the grams of H2O (2.39).
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