text = ""
while text != "quit":
    text = raw_input("Please enter a chemical formula (or 'quit' to exit): ")
    if text == "quit":
        print "...exiting program"
    elif text == "H2O":
        print "Water"
    elif text == "NH3":
        print "Ammonia"
    elif text == "CH3":
        print "Methane"
    else:
        print "Unknown compound"