Final answer:
For an older residential split system air conditioner, R-22 refrigerant is most likely to be found. R-22 was common before more eco-friendly options became prevalent, and unlike the alternative refrigerants listed, it was specifically used in residential air conditioning.
Step-by-step explanation:
If you are servicing an older residential split system air conditioner, you would most likely expect to find the system charged with R-22 refrigerant. This refrigerant was commonly used in older systems before more environmentally friendly alternatives were introduced. The other refrigerants listed, R-11, R-12, and R-502 have specific applications and were generally not used in residential split system air conditioners. R-12 was typically utilized in automotive applications, and both R-11 and R-502 were used in industrial or commercial refrigeration systems. It should be noted that since January 2020, production and import of R-22 have been banned in the United States under the Montreal Protocol due to its ozone-depleting properties, although servicing existing equipment with recycled or stockpiled R-22 is still permissible.