Final answer:
Under isothermal compression, the temperature of an ideal gas remains constant. In this case, the gas temperature stays at 273 K after the volume is reduced by half.
Step-by-step explanation:
The question asks about the temperature of an ideal gas after it undergoes isothermal compression in a sealed container. If an ideal gas at 273 K has its volume decreased by one-half (50%) under isothermal conditions, the temperature remains constant because 'isothermal' means 'at a constant temperature'. Therefore, even after compression, the temperature of the gas remains 273 K.