15. Write the name of given formula
V. m. 3KT/m
Your answer
Answers
Answer:
How about a simple, straightforward problem?
Compute the rms speed of an oxygen molecule at room temperature.
Use the results of part a. to determine the rms speed of a hydrogen molecule at room temperature.
Use the results of part b. to determine the rms speed of a mercury atom at 1200 K.
Use the formula. Recall that oxygen is a diatomic molecule in everyday situations. Let's not go nuts here with precision. Just use the approximate molecular mass of oxygen (2 × 16 u = 32 u) and the approximate value of room temperature (300 K). Let's try calculating it both ways: first with the mass of a molecule…
vO2 = √ 3kT
m
vO2 = √ 3(1.38 × 10−23 J/K)(300 K)
(32 u)(1.66 × 10−27 kg/u)
vO2 = 483.5 m/s
and then with the mass of a mole…
vO2 = √ 3RT
M
vO2 = √ 3(8.31 J/molK)(300 K)
(0.032 kg/mol)
vO2 = 483.4 m/s
The two answers are slightly different in the fourth significant digit, but I said to be reasonable with the precision. Let's just say the answer is…
vO2 = 480 m/s
Exploit the simple ratio of the two molecular masses. Oxygen is 16 times heavier than hydrogen on a per atom or per molecule comparison (since both gases are diatomic in our everyday lives). RMS speed is inversely proportional to the square root of mass (molecular or molar). This means the rms speed of hydrogen should be √16 = 4 times faster. If you would like to see the mathematical reasoning presented formally, here it is…
vH2 = √3kT/mH2
vO2 √3kT/mO2
vH2 = √ mO2
vO2 mH2
vH2 = √ 32 u = 4
vO2 2 u
vH2 = 4
vO2
vH2 = 4 vO2
vH2 = 4(480 m/s)
vH2 = 1,920 m/s
Another question with rigged numbers. The atomic mass of mercury (200 u) is 100 times that of molecular hydrogen (2 u). This difference reduces the speed by
1
√100
=
1
10
. In a similar vein, the temperature of these mercury atoms is 4 times that of the hydrogen molecules in part b. This change raises the rms speed by a factor of √4 = 2. Combining both changes gives a new rms speed that's
2
10
of the old one. Again, if you would like to see the mathematical reasoning presented formally, here it is…
vHg = √3kTHg/mHg
vH2 √3kTH2/mH2
vHg = √ THgmH2
vH2 TH2mHg
vHg = √ (1200 K)(2 u)
vH2 (300 K)(200 u)
vHg = 2 = 1
vH2 10 5
vHg = ⅕vH2
vHg = ⅕(1,920 m/s)
vHg = 384 m/s
Explanation: