Final answer:
The capacity of a portable CO₂ extinguisher by weight is not given, but it is important to understand the critical temperature of CO₂, which is 31°C. Above this temperature, the CO₂ cannot liquefy, potentially leading to dangerous increases in pressure if stored improperly.
Step-by-step explanation:
The maximum capacity, by weight, of a portable CO₂ extinguisher is not explicitly given in the information provided. However, understanding the behavior of CO₂ in an extinguisher can offer insight into safety considerations. For a CO₂ extinguisher at 21°C and 20.0 atm pressure, an increase in temperature to 71.1°C can significantly increase internal pressure due to the gas laws. With CO₂, the critical temperature is 31°C, above which it cannot liquefy regardless of pressure. Therefore, storing extinguishers at temperatures above this critical point can lead to dangerous increases in internal pressure, possibly exceeding the capacity of the container.