Physical Chemistry Problem, Evacuated tube - where does reaction take place?

[MeMoses]

Homework Statement

Two balls, one soaked in concentrated HCl and the other in NH4OH are placed at the left and right ends of a 1m long evacuated glass tube, respectively. HCl and ammonia vaporize and travel down the tube, reacting to form a white ring where they meet. Where does the ring form?

Homework Equations

The Attempt at a Solution

I'm not sure what to do with this. At first I thought it dealt with diffusion but I couldn't get anything useful (unless I'm overlooking something). Then I realized the tube is empty, so can I just use the average speed o the gas molecules and see where the gasses meet or will that not work? Speed is not a vector though, so do I have to find the velocity of the net movement of the gases towards each other? If so how would I calculate that?
Thanks

 

[mfb]

The speed of the molecules is somewhere at ~500m/s, which means that they need ~2ms to travel through the pipe in vacuum. The preparation of your experiment will need much more time.
One of the gases might evaporate quicker (and therefore in a larger amount), and shift the reaction area towards the other side. That is just a guess, however.