Re: What is the average speed (in MPH) of a typical water molecule?

Hi Bob,

We can calculate this using the root-mean-square velocity formula for gases. The RMS velocity is the square-root of the average velocity-squared of a molecule in the gas form. Although this is not exactly the same as average velocity, it will be close enough for our estimation purposes. RMS velocity = sqrt(3RT/M), where R = gas constant, T = temperature, M = molar mass of the molecule. Plugging in the values for water at 37C gives ~650m/s, which equals to be about 1500 miles per hour.

However, we must account for the fact that molecules in the liquid phase move slower than in the gas. According to this site, the atoms (as unsticky as you can get) move about 8 times slower as a liquid. In that case, a safe approximation for water is probably about 5 times slower, giving ~300 miles per hour average for each water molecule.

Good luck with the course!
Kathy