Gas Velocity

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This is the equation you need to use:

v = √(3RT/M)

You may, if you wish, read more about the above equation here.

The basic idea is that, if you consider each gas molecule's velocity (which has components of both speed and direction), the average velocity of all gas molecules in a sample is zero. That stems from the fact that the gas molecules are moving in all directions in a random way and each random speed in one direction is cancelled out by a molecule randomly moving in the exact opposite direction, with the exact same speed (when the gas sample is considered in a random way).

So, what you do is square each velocity, which gets rid of the negative sign (molecules moving in one way have a positive sign for their direction, those moving in the opposite direction have a negative direction). You then average all of these squared values and take the square root of that average.

It's called a 'root mean square' and technically, it is a speed, not a velocity. However, in chemistry, we ignore the distinction between speed and velocity and use velocity.

It upsets the physics guys. Oh Well.

Some necessary information:

R = 8.31447 J mol¯11

M = the molar mass of the substance, expressed in kilograms

Click this link for a discussion on video about how the units cancel in v = √(3RT/M)

Problem #1: What is the ratio of the average velocity of krypton gas atoms to that of nitrogen molecules at the same temperature and pressure?

Solution path one:

1) Let us determine the velocity of Kr atoms at 273 K:

v = √(3RT/M)

v = √[[(3) (8.31447) (273)] / (0.083798)]

v = 285 m/s

2) Let us determine the velocity of N2 atoms at 273 K:

v = √(3RT/M)

v = √[[(3) (8.31447) (273)] / (0.028014)]

v = 493 m/s

3) Let us expresss the ratio both ways:

N2 : Kr ratio = 493 / 285 = 1.73 to 1

Kr : N2 ratio = 285 / 493 = 0.578 to 1

Solution path two:

Please note that the 3RT term is common to both velocity calculations above. 3R is a constant and TKr = TN2. That means 3RT can be cancelled out if we are being asked for a ratio. Let's do the Kr : N2 ratio:

vKr / vN2 = √[3RTKr / 0.083798] / [3RTN2 / 0.028014]

vKr / vN2 = √[0.028014 / 0.083798]

vKr / vN2 = 0.578

Problem #2: At the same temperature, which molecule travels faster, O2 or N2? How much faster?


1) Oxygen velocity @ 273 K:

v = √(3RT/M)

v = √[[(3) (8.31447) (273)] / (0.0159994)]

v = 652.4 m/s

2) Nitrogen velocity @ 273 K:

v = √(3RT/M)

v = √[[(3) (8.31447) (273)] / (0.028014)]

v = 493 m/s

3) Nitrogen moves faster. How much faster?

652.4 / 493 = 1.3233

Nitrogen moves slightly more than 1.3 times as fast as oxygen, at the same temperature.

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