Combined Gas law
Calculations
Standard Temperature and
Pressure (STP)
Standard Pressure
760 mmHg= 760 torr =1 atm = 101.3 kPa
Standard Temperature
0°C = 273 K
Standard Ambient
Temperature and Pressure
(SATP)
• Normal conditions under which we live
Temperature = 25°C
Pressure = 100 kPa
Combined Gas Law
• Combines Boyle’s Law, Charles’ Law
and Gay-Lussac’s Law
• Possible to calculate situations in
which three variables (pressure,
temperature and volume) change at
the same time.
Mathematically,
PiVi
Ti
=
PfVf
Tf
Sandra is having a birthday party on a mild winter’s
day. The weather changes and a higher pressure
(103.0 kPa) cold front (-25°C) rushes into town. The
original air temperature was –2.0 °C and the pressure
was 100.8 kPa. What will happen to the volume of the
4.2L ballons that were tied to the front of the
house?
Step 1: What are you trying to
determine?
What will happen to the volume of the 4.2L ballons that were
tied to the front of the house?
Vf
• Step 2: Identify what is given:
•
Vi = 4.2L
•
Ti = -2.0 °C
Tf = -25 °C
•
Pi = 100.8 kPa
Pf = 103.0 kPa
• Step 3:
• Convert temperatures from °C to K
• Ti = -2 °C
Tf = -25 °C
• = -2 + 273
= -25 + 273
• = 271 K
= 248 K
• Step 4: Use equation to determine final
volume:
PiVi
Ti
PfVf
=
Tf
(100.8kPa)(4.2L)
271K
(100.8kPa)(4.2L)(248K)
(271K)(103.0kPa)
=
(103.0 kPa)(Vf)
248K
=
Vf
Vf = 3.76 L
= 3.8 L
An automated instrument has been
developed to help drug-research chemists
determine the amount of nitrogen in a
compound.
In an analysis of 39.8 mg of caffeine using
this instrument, 10.1 mL of N2 gas is
produced at 23°C and 746 torr. What must
the new temperature of nitrogen be, in °C, if
the volume is increased to 12.0 mL and the
pressure is increase to 780 torr?
Step 1: What are you trying to
determine?
What must the new temperature of nitrogen be, in
°C ….
Tf
• Step 2: Identify what is given:
•
Vi = 10.1 mL
Vf = 12.0 mL
•
Ti = 23 °C
•
Pi = 746 torr
Pf = 780 torr
• Step 3:
• Convert temperatures from °C to K
• Ti = 23 °C
•
= 23 + 273
•
= 297 K
Step 4: Use equation to determine final
volume:
PiVi
PfVf
Ti
Tf
To isolate Tf, flip both sides of the equation,
the multiply by Pf and Vf
Ti PfVf = Tf
Pi Vi
(296K)(780 torr)(12.0 mL) = T
f
(746 torr)(10.1 mL)
Tf = 368 K
Step 5: Convert the temperature back to °C
Tf = 368 –273
= 95°C