Mixtures and diffusion

You will write functions for Dalton's law and Graham's law. One finds the partial pressure of every gas in a mixture. One finds the pressure of a gas collected over water. One compares the rates of two gases. One finds where two gases meet in a tube.

The idea. The partial pressure of a gas is its mole fraction times the total pressure. A gas over water has the pressure of the total minus the vapour pressure of water. By Graham's law, $\dfrac{r_1}{r_2} = \sqrt{\dfrac{M_2}{M_1}}$. If gas 1 enters at one end of a tube and gas 2 at the other end, the distances travelled are in the ratio of the rates.

Your tasks.

1. Complete partial_pressures(moles, total) so that it returns a dictionary with the partial pressure of each gas.
2. Complete dry_pressure(p_total, p_water).
3. Complete rate_ratio(m1, m2) so that it returns the rate of gas 1 divided by the rate of gas 2.
4. Complete meeting_point(m1, m2, length) so that it returns the distance of the meeting point from the end where gas 1 enters.