10.9: While Loops

 

In the code below enter a number less that 2 three times, and then one greater than 2.  

fw=float(input("Formula Weight of molecue: "))

while fw <= 2:
    print(f' This is impossible as {fw} is smaller than hydrogen')
    fw=float(input("Input another Formula Weight of the molecule: "))
else: 
   print("you may have a real molecule")
print("you are out of loop")

Hello world!

Notice you stayed in the loop as long as you gave an unrealistic molar mass, and so the program prevented you from inputing data that could not  be real.  The moment you inputed a value greater than 2, the loop terminated, and you could have used that variable in a subsequent calculation.