pressure exerted on the sides of its container is directly proportional to its absolute temperature. As a mathematical equation, Gay-Lussac's law is written as either: , or , or , where P is the pressure, T is the absolute temperature, and k is another proportionality constant. Avogadro's law Avogadro's law (hypothesized in 1811) states that at a constant temperature and pressure, the volume occupied by an ideal gas is directly proportional to the number of molecules of the gas present in the container. This gives rise to the molar volume of a gas, which at STP (273.15 K, 1 atm) is about 22.4 L. The relation is given by , or where n is equal to the number of molecules of gas (or the number of moles of gas). Combined and ideal gas laws The Combined gas law or General Gas Equation is obtained by combining Boyle's Law, Charles's law, and Gay-Lussac's Law. It shows the relationship between the pressure, volume, and temperature for a fixed mass (quantity) of gas: This can also be written as: With the addition of Avogadro's law, the combined gas law develops into the ideal gas law: where P is pressure V is volume n is the number of moles R is the universal gas constant T is temperature (K) The proportionality constant, now named R, is the universal gas constant with a value of 8.3144598 (kPa∙L)/(mol∙K). An equivalent formulation of this law is: where P is the pressure V is the volume N is the number of gas molecules kB is the Boltzmann constant (1.381×10−23J·K−1 in SI units) T is the temperature (K) These equations are exact only for an ideal gas, which neglects various intermolecular effects (see real gas). However, the ideal gas law is a good approximation for most gases under moderate pressure and temperature. This law has the following important consequences: If temperature and pressure are kept constant, then the volume of the gas is directly proportional to the number of molecules of gas. If the temperature and volume remain constant, then the pressure of the gas changes is directly proportional to the number of molecules of gas present. If the number of gas molecules and the temperature remain constant, then the pressure is inversely proportional to the volume. If the temperature changes and the number of gas molecules are kept constant, then either pressure or volume (or both) will change