- #1
lc99
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- 3
Homework Statement
Let's say, P = 1.00 atm
and T =273K
and density of air at STP = 1.29 g/L
Homework Equations
The Attempt at a Solution
I'm not too sure...
PV = nRT
n = PV/RT
= P/RT = 1/(0.0821*273) *1.29 g/L ?
Borek said:PV=nRT alone is not enough, you need air composition as well.
For any gas: assume 1 m3 of gas, can you calculate number of moles? Mass? That will give you density instantly.
For air: air has no molar mass, as it is a mixture, but every mixture has its apparent molar mass (behaves as a gas of such), one that can be calculated as a weighted average of molar masses of the components.
Alternatively: assume 1 m3 of gas. Knowing the air composition, can you calculate volume occupied by the nitrogen? Or its partial pressure? Of other gases?
figured out how to get the moles of air.. so, the weighted avg of molar mass would be the a lot of gases wouldn't it?Borek said:I gave you plenty of hints, have you tried to use them?
You can use this equation at any temperature and pressure at which the ideal gas law is a good approximation for your particular gas.lc99 said:i
figured out how to get the moles of air.. so, the weighted avg of molar mass would be the a lot of gases wouldn't it?
i feel like I am missing some information. would finding the molar mass from the density of air at stp help to find molar mass of air?
so M = dRT/P at stp = 28.913 g/mol
so i can use the equation d = PM/RT for any temperature and pressure?
lc99 said:the weighted avg of molar mass would be the a lot of gases wouldn't it?
Borek said:Nope, enough to treat the air as a mixture of just nitrogen and oxygen, other ones are in minute quantities and can be safely ignored.
The formula for calculating the density of air using the ideal gas law is D = (P * M) / (R * T), where D is the density, P is the pressure, M is the molar mass of air, R is the gas constant, and T is the temperature in Kelvin.
The value of the gas constant used in the ideal gas law is R = 0.0821 L * atm / mol * K, where L is liters, atm is atmospheres, mol is moles, and K is Kelvin.
To use the ideal gas law, you will need to convert the units of pressure to atmospheres, volume to liters, and temperature to Kelvin. To convert from Celsius to Kelvin, add 273.15 to the Celsius temperature. To convert from Fahrenheit to Kelvin, first convert to Celsius using the formula (F - 32) * (5/9), then add 273.15.
No, the ideal gas law is only valid for systems with constant temperature and pressure. The density of air changes with altitude due to changes in pressure and temperature, so the formula may not accurately reflect the density at higher altitudes.
The molar mass of air used in the ideal gas law is approximately 28.97 g/mol. This value can vary slightly depending on the composition of air, but is generally used as an average value for calculations.