When I was visiting my relatives in southeast New Mexico last week, I was asked to solve a chemistry problem with real-life impact. All my reasoning came from freshman-level chemistry.
The thing was my grandma had run out of potassium chloride solution that she needs every day. For some reason the pharmacy didn't have it or something. To make up for it she was eating bananas but they didn't have much of an effect and she was feeling sick. So I was asked if there was something in the pantry they could try.
Ok, any guesses? Potassium chloride in the kitchen?
I remembered that some salt substitutes were potassium (instead of sodium) chloride. They had one, and while there were a few trace ingredients, it was mostly potassium chloride. So then I just had to figure out how to make a 10% solution of that.
The useful rule of thumb is that water weighs 1 gram per milliliter, so a 100% solution would be 1 gram per milliliter of water. (I'm skipping a few things here but it's close enough.) Of course, the salt was measured by volume, not weight, but on the back it said 1/6 tsp = 1 gram. I figured 30mL of a 10% solution would require 3 grams of salt, so would take 3/6 tsp or 1/2 tsp of salt substitute.
The uncertainties? Those trace elements are weird, and I know too much potassium can hurt or even kill a person (potassium is part of the lethal injection regime), but when the answer came out to be just 1/2 a teaspoon of salt, I felt a little better. But still a bit nervous.
Anyway, the next day grandma was feeling much better. Looks like the chemistry did the trick. And now I have a "real-life application" calculation problem for the next time I teach freshman chem.