SME
Grade 10 Science · Problem type
Effusion: Graham’s law
Use this for “Helium (4.00 g/mol) and argon (39.95 g/mol) leak from one tank. Which escapes faster, and how many times as fast?”
Diagram
rate
of
He
rate
of
Ar
=
√(
39.95
4
)
=
3.16
The lighter He moves faster and escapes 3.16 times as fast as Ar.
At one temperature every gas has the same average kinetic energy, so lighter molecules move faster.
This is an example. Type your own values to start a new problem.
r
=
M₂ (g/mol)
M₁ (g/mol)
Formulas
r
=
√
M₂
M₁
3.16
=
√
39.95/4
Assumptions
•
Both gases are at the same temperature, so their molecules have the same average kinetic energy.
•
Lighter molecules move faster, so they find the pinhole more often.
•
rate₁ ÷ rate₂ = √(M₂ ÷ M₁).
Step-by-step
WE KNOW
M₁ = 4 g/mol, M₂ = 39.95 g/mol
FIND
how many times as fast gas 1 escapes (r)
Step 1 · Find how many times as fast gas 1 escapes (r)
Use r = √(M₂/M₁)
Graham’s law: the rate goes as 1 over the square root of the molar mass.
r
=
√
39.95/4
r
=
√
9.9875
r
=
3.16
Check
Put the numbers back in. Both sides should match.
3.16
≈
√
39.95/4
✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
Related lessons
Gases and the gas laws (PV = nRT)
Main lesson
Boyle’s law: squeezing a gas
Use this for a gas squeezed or let out at one temperature: “6 L at 1 atm is pressed into 3 L. What is the pressure?”
Charles’s law: heating a gas
Use this for a gas warmed or cooled at one pressure: “2.00 L at 300 K is heated to 450 K. What is its volume?”
Gay-Lussac’s law: a sealed, rigid container
Use this for a gas heated in a rigid container: “A tire at 2.00 atm and 280 K warms to 308 K. What is the pressure?”
The combined gas law
Use this when pressure, volume and temperature all change: “5.00 L at 1.00 atm and 300 K goes to 2.00 atm and 360 K. What is the volume?”
Dalton’s law of partial pressures
Use this for a mixture of gases: the total pressure from each gas’s partial pressure, and one gas’s mole fraction.