Boyle's Law Cylinder — FRCA Primary MCQ

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ModeratePhysics & Clinical MeasurementBoyle's Law CylinderFRCA Primary

A size E oxygen cylinder contains approximately 680 L of oxygen when full, at which point its pressure gauge reads 13,700 kPa. Assuming the cylinder temperature remains constant, approximately how much oxygen remains when the gauge reads 6,850 kPa?

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Correct answer: C — 340 L

The correct answer is C, 340 L. Oxygen is stored as compressed gas, and for a fixed-volume cylinder at constant temperature the usable gas content is approximately proportional to the gauge pressure. The indicated pressure has fallen from 13,700 kPa to 6,850 kPa, exactly one-half of the full reading. Therefore, the remaining content is 680 × 1/2 = 340 L. This proportional relationship applies to oxygen because it remains gaseous at ordinary cylinder temperatures. Option E is the full-cylinder content; A is twice the full content. Option A represents one-quarter full, while D represents three-quarters full. In practice, cylinder contents and duration calculations remain approximate because temperature, gauge accuracy and residual pressure can affect the available volume.

Reference: Royal Cornwall Hospitals NHS Trust. Safe Transfer of Patients Between Care Areas or Hospitals Policy, Appendix 6: Oxygen Cylinder Contents and Flow Rates. Version 5.1, August 2025. https://doclibrary-rcht.cornwall.nhs.uk/DocumentsLibrary/RoyalCornwallHospitalsTrust/Clinical/CorporateClinical/SafeTransferOfPatientsBetweenCareAreasOrHospitalsPolicy.pdf