The total pressure (vapor pressure) of a mixture of liquefied gas at a given temperature depends on the percentage and vapor pressure of its constituent gases. At 100 degrees Fahrenheit, pure normal butane has a pressure of about 37 pounds per square inch and pure propane has a pressure of about 172 pounds per square inch. Therefore, a mixture of normal butane and propane at 100 degrees Fahrenheit has a pressure between 37 and 172 pounds per square inch, depending on the percentage of hydrocarbons that make it up.
In practice, commercial liquefied petroleum gas is not just a mixture of butane and propane, but also contains other hydrocarbons that have higher vapor pressures, such as ethane, ethylene, and propylene. Therefore, depending on the amount and type of hydrocarbons, it has a pressure of far more than 173 pounds per square inch.
All hydrocarbons that make up liquefied petroleum gas are heavier than air. This is a very important issue from a safety point of view, because if liquefied petroleum gas is released, especially in free quantities, it will try to accumulate in round and low places such as basements, ponds and other pits on the ground, and may catch fire before spreading into the air.
Liquefied petroleum gas is practically colorless and odorless, and therefore odorous substances are added to it so that in case of leakage, its presence can be felt in the air by inhalation.