Estimate the Answer

Use the units to check that they’ve actually done the right calculation.  The other thing is when they’ve got the mole concept and they’re applying it, try to get students to do an estimation of the answer.  So are they estimating something that’s very large? For example, if they need to know how many atoms of lead are in a particular volume of lead, or are they looking for a mass that they could walk into the lab and weigh, that is, several grams not several nanograms, which of course would be crazy.  Or are they looking to quantify the mass of just one particle or one molecule and you’re adding up atomic masses to generate a molecular mass. It’s partly order of magnitude but it’s also recognising how to bring all the various things together so that they’re coherent and give you the answer.