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Updated on February 03, 2020 What is the density of air at STP? In order to answer the question, you need to understand what density is and how STP is defined. Key Takeaways: Density of Air at STP
The density of air is the mass per unit volume of atmospheric gases. It is denoted by the Greek letter rho, ρ. The density of air, or how light it is, depends on the temperature and pressure of the air. Typically, the value given for the density of air is at STP (standard temperature and pressure). STP is one atmosphere of pressure at 0 degrees C. Since this would be a freezing temperature at sea level, dry air is less dense than the cited value most of the time. However, air typically contains a lot of water vapor, which would make it denser than the cited value. The Density of Air ValuesThe density of dry air is 1.29 grams per liter (0.07967 pounds per cubic foot) at 32 degrees Fahrenheit (0 degrees Celsius) at average sea-level barometric pressure (29.92 inches of mercury or 760 millimeters).
Affect of Altitude on DensityThe density of air decreases as you gain altitude. For example, the air is less dense in Denver than in Miami. The density of air decreases as you increase temperature, providing the volume of the gas is allowed to change. As an example, air would be expected to be less dense on a hot summer day versus a cold winter day, providing other factors remain the same. Another example of this would be a hot air balloon rising into a cooler atmosphere. STP Versus NTPWhile STP is standard temperature and pressure, not many measured processes occur when it's freezing. For ordinary temperatures, another common value is NTP, which stands for normal temperature and pressure. NTP is defined as air at 20 degrees C (293.15 K, 68 degrees F) and 1 atm (101.325 kN/m2, 101.325 kPa) of pressure. The average density of air at NTP is 1.204 kg/m3 (0.075 pounds per cubic foot). Calculate the Density of AirIf you need to calculate the density of dry air, you can apply the ideal gas law. This law expresses density as a function of temperature and pressure. Like all gas laws, it is an approximation where real gases are concerned but is very good at low (ordinary) pressures and temperatures. Increasing temperature and pressure adds error to the calculation. The equation is: ρ = p / RT where:
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The density of air or atmospheric density, denoted ρ, is the mass per unit volume of Earth's atmosphere. Air density, like air pressure, decreases with increasing altitude. It also changes with variation in atmospheric pressure, temperature and humidity. At 101.325 kPa (abs) and 20 °C (68 °F), air has a density of approximately 1.204 kg/m3 (0.0752 lb/cu ft), according to the International Standard Atmosphere (ISA). At 101.325 kPa (abs) and 15 °C (59 °F), air has a density of approximately 1.225 kg/m3 (0.0765 lb/cu ft), which is about 1⁄800 that of water, according to the International Standard Atmosphere (ISA).[citation needed] Pure liquid water is 1,000 kg/m3 (62 lb/cu ft). Air density is a property used in many branches of science, engineering, and industry, including aeronautics;[1][2][3] gravimetric analysis;[4] the air-conditioning[5] industry; atmospheric research and meteorology;[6][7][8] agricultural engineering (modeling and tracking of Soil-Vegetation-Atmosphere-Transfer (SVAT) models);[9][10][11] and the engineering community that deals with compressed air.[12] Depending on the measuring instruments used, different sets of equations for the calculation of the density of air can be applied. Air is a mixture of gases and the calculations always simplify, to a greater or lesser extent, the properties of the mixture. Temperature[edit]Other things being equal, hotter air is less dense than cooler air and will thus rise through cooler air. This can be seen by using the ideal gas law as an approximation. Dry air[edit]The density of dry air can be calculated using the ideal gas law, expressed as a function of temperature and pressure:[citation needed] where:[citation needed] Therefore:
The following table illustrates the air density–temperature relationship at 1 atm or 101.325 kPa:[citation needed] Effect of temperature on properties of air
Humid air[edit]Effect of temperature and relative humidity on air density The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear counter-intuitive. This occurs because the molar mass of water vapor (18 g/mol) is less than the molar mass of dry air[note 2] (around 29 g/mol). For any ideal gas, at a given temperature and pressure, the number of molecules is constant for a particular volume (see Avogadro's Law). So when water molecules (water vapor) are added to a given volume of air, the dry air molecules must decrease by the same number, to keep the pressure or temperature from increasing. Hence the mass per unit volume of the gas (its density) decreases. The density of humid air may be calculated by treating it as a mixture of ideal gases. In this case, the partial pressure of water vapor is known as the vapor pressure. Using this method, error in the density calculation is less than 0.2% in the range of −10 °C to 50 °C. The density of humid air is found by: [13]where: , density of the humid air (kg/m3), partial pressure of dry air (Pa), specific gas constant for dry air, 287.058 J/(kg·K), temperature (K), pressure of water vapor (Pa), specific gas constant for water vapor, 461.495 J/(kg·K), molar mass of dry air, 0.0289652 kg/mol, molar mass of water vapor, 0.018016 kg/mol, universal gas constant, 8.31446 J/(K·mol)The vapor pressure of water may be calculated from the saturation vapor pressure and relative humidity. It is found by: where: , vapor pressure of water, relative humidity (0.0–1.0), saturation vapor pressureThe saturation vapor pressure of water at any given temperature is the vapor pressure when relative humidity is 100%. One formula is Tetens' equation from[14] used to find the saturation vapor pressure is: where: , saturation vapor pressure (hPa), temperature (°C)See vapor pressure of water for other equations. The partial pressure of dry air is found considering partial pressure, resulting in: Where simply denotes the observed absolute pressure. Variation with altitude[edit]Standard atmosphere: p0 = 101.325 kPa, T0 = 288.15 K, ρ0 = 1.225 kg/m3 Troposphere[edit]To calculate the density of air as a function of altitude, one requires additional parameters. For the troposphere, the lowest part (~10 km) of the atmosphere, they are listed below, along with their values according to the International Standard Atmosphere, using for calculation the universal gas constant instead of the air specific constant: , sea level standard atmospheric pressure, 101325 Pa, sea level standard temperature, 288.15 K, earth-surface gravitational acceleration, 9.80665 m/s2, temperature lapse rate, 0.0065 K/m, ideal (universal) gas constant, 8.31446 J/(mol·K), molar mass of dry air, 0.0289652 kg/molTemperature at altitude meters above sea level is approximated by the following formula (only valid inside the troposphere, no more than ~18 km above Earth's surface (and lower away from Equator)): The pressure at altitude is given by: Density can then be calculated according to a molar form of the ideal gas law: where: , molar mass, ideal gas constant, absolute temperature, absolute pressureNote that the density close to the ground is It can be easily verified that the hydrostatic equation holds: Exponential approximation[edit]As the temperature varies with height inside the troposphere by less than 25%, and one may approximate: Thus: Which is identical to the isothermal solution, except that Hn, the height scale of the exponential fall for density (as well as for number density n), is not equal to RT0/gM as one would expect for an isothermal atmosphere, but rather: Which gives Hn = 10.4 km. Note that for different gasses, the value of Hn differs, according to the molar mass M: It is 10.9 for nitrogen, 9.2 for oxygen and 6.3 for carbon dioxide. The theoretical value for water vapor is 19.6, but due to vapor condensation the water vapor density dependence is highly variable and is not well approximated by this formula. The pressure can be approximated by another exponent: Which is identical to the isothermal solution, with the same height scale Hp = RT0/gM. Note that the hydrostatic equation no longer holds for the exponential approximation (unless L is neglected). Hp is 8.4 km, but for different gasses (measuring their partial pressure), it is again different and depends upon molar mass, giving 8.7 for nitrogen, 7.6 for oxygen and 5.6 for carbon dioxide. Total content[edit]Further note that since g, Earth's gravitational acceleration, is approximately constant with altitude in the atmosphere, the pressure at height h is proportional to the integral of the density in the column above h, and therefore to the mass in the atmosphere above height h. Therefore the mass fraction of the troposphere out of all the atmosphere is given using the approximated formula for p: For nitrogen, it is 75%, while for oxygen this is 79%, and for carbon dioxide, 88%. Tropopause[edit]Higher than the troposphere, at the tropopause, the temperature is approximately constant with altitude (up to ~20 km) and is 220 K. This means that at this layer L = 0 and T = 220 K, so that the exponential drop is faster, with HTP = 6.3 km for air (6.5 for nitrogen, 5.7 for oxygen and 4.2 for carbon dioxide). Both the pressure and density obey this law, so, denoting the height of the border between the troposphere and the tropopause as U: Composition[edit]Composition of dry atmosphere, by volume[▽ note 1][▽ note 2]
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What is normal air density?At sea level (z = 0), average pressure is about 101 kiloPascals (kPa), and average air density is about 1.225 kg/m3.
What is the density of air at STP in Kg m 3?Therefore: At IUPAC standard temperature and pressure (0 °C and 100 kPa), dry air has a density of approximately 1.2754 kg/m3.
What is the density of air at 1 atm and 25 C?Engineering Physics. |