Which of the following is NOT considered a standard state under standard temperature and pressure (STP) conditions?

• A. Liquid water at 1 atm and 273.15 K
• B. Carbon dioxide gas at 2 atm and 273.15 K
• C. Nitrogen gas at 1 atm and 273.15 K
• D. Oxygen gas at 1 atm and 273.15 K

Answer: (B)

Verified step by step guidance

1. Understand the definition of standard state conditions: Standard state refers to the most stable form of a substance at 1 atm pressure and a specified temperature, usually 25°C (298.15 K), but sometimes 0°C (273.15 K) is used for STP. The key point is that the pressure must be 1 atm.
2. Identify the pressure and temperature conditions given for each substance in the problem. Note that standard state requires a pressure of exactly 1 atm and the given temperature (273.15 K in this case).
3. Check each option against the standard state criteria: Liquid water at 1 atm and 273.15 K is standard state because water is liquid at 1 atm and 273.15 K; Nitrogen gas at 1 atm and 273.15 K is standard state; Oxygen gas at 1 atm and 273.15 K is standard state.
4. Look at the option with carbon dioxide gas at 2 atm and 273.15 K. Since the pressure is 2 atm, which is not 1 atm, this does not meet the standard state pressure condition.
5. Conclude that the substance not considered in its standard state under the given conditions is carbon dioxide gas at 2 atm and 273.15 K because its pressure deviates from the required 1 atm standard state pressure.