Final answer:
A ventilation pressure of 20 cmH2O can force air into the stomach during facemask ventilation, but excessive pressure should be avoided.
Step-by-step explanation:
In the scenario of facemask ventilation, the pressure required to force air into the stomach would depend on the resistance encountered by the airway. Generally, a ventilation pressure of 20 cmH2O can often force air into the stomach, especially if the airway resistance is low. However, it is essential to highlight that it is crucial to prevent excessive pressure as it can lead to gastric insufflation, which can be harmful to the patient.