There are many reasons why calculations involving amount of substance and concentration cause more difficulty than they ought to at i/GCSE / IB / A level / pre-U / AP.  One of the most common stems from a poor layout for solutions to problems.  If robust and correct procedures are not adopted at the outset, then a candidate has no foundation upon which to rely when two-step and multi-step problems are encountered.  Such difficulties are only remedied by going right back to the beginning, something poorly equipped students are usually reluctant to do.  And doing so is very time consuming and inefficient.