Make a Molecular Weight calculator. The user enters for instance the formula for salt NaCl and the program looks up the atomic weight of sodium and chlorine and adds them up.

A dictionary with element:weight pairs would make it easy. You also have to design some kind of parser that figures out molecules like water H2O. Here you have to add up hydrogen twice and oxygen once.

Things get a little more challenging for the parser with toluene, since the user could enter C7H8 or C6H5CH3. Elements like silicon Si have to be entered that way and not as SI, since your parser would look at it as a combination of sulfur S and iodine I.

Since the right data is already there, you can expand the program to do elemental analysis, giving the percent of each element in the formula. Happy coding!