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Why does IUPAC suggest using standard pressure and temperature reference conditions when measuring physical and chemical properties of matter?

STP in chemistry is the abbreviation for Standard Temperature and Pressure. STP most commonly is used when performing calculations on gases, such as gas density. The standard temperature is 273 K (0° Celsius or 32° Fahrenheit) and the standard pressure is 1 atm pressure. This is the freezing point of pure water at sea level atmospheric pressure. At STP, one mole of gas occupies 22.4 L of volume (molar volume).

Note the International Union of Pure and Applied Chemistry (IUPAC) applies a more stringent standard of STP as a temperature of 273.15 K (0 °C, 32 °F) and an absolute pressure of exactly 100,000 Pa (1 bar, 14.5 psi, 0.98692 atm). This is a change from their earlier standard (changed in 1982) of 0 °C and 101.325 kPa (1 atm).

STP is the abbreviation for Standard Temperature and Pressure. However, the "standard" is defined differently by various groups.
STP values are most often cited for gases because their characteristics change dramatically with temperature and pressure.
One common definition of STP is a temperature of 273 K (0° Celsius or 32° Fahrenheit) and the standard pressure of 1 atm. Under these conditions, one mole of a gas occupies 22.4 L.
Because the standard varies by industry, it's good practice to state temperature and pressure conditions for measurements and not just say "STP."