Something like the RSC's Problem solving tutor would be excellent here but there is not an example using a buffer calculation. Therefore, a video of eg Tim going through a buffer calculation example in this style would work well, or an animation done in a similar style to the problem solving tutor (I think the video maybe the simplest to make, assuming the teacher is willing!)

Key points:

How to help students problem solve. Scaffolding

Know, LIST, what you know.

And what you don't know

eg total volume? (often forgotten)