Solving Gas Mixture Problem: Calculate Number of Moles

In summary: I guess the instructor overlooked the fact that each gas has its own partial pressure and therefore should have a different number of moles. In summary, the problem is asking for the number of moles present of a gas in a gaseous mixture containing 2.75 L of helium, some argon at 1.10 atm, and some neon at 25ºC. The ideal gas law, PV=nRT, can be used to calculate the number of moles for each gas, with the partial pressure of argon being the only given information. However, it is unclear which gas the instructor intended for the calculation to be done for, as each gas has its own partial pressure and should have a different number of moles.
  • #1
Bohrok
867
0

Homework Statement



A gaseous mixture contains 2.75 L of helium, some argon at 1.10 atm, and some neon at 25ºC. For which gas can the number of moles be calculated? Calculate the number of moles present of that gas.
(This is exactly what the problem asks.)

Homework Equations



PV = nRT

The Attempt at a Solution



I'm trying to help some chemistry students with this problem. Either the problem statement is a little ambiguous, or I'm missing something simple.
I believe the helium would occupy 2.75 L by itself, the 1.10 atm of argon is the partial pressure of the mixture for just argon, and the 25ºC (or 298 K) applies to the whole mixture. So far is this correct?

This is what I get next:

PHe(2.75 L) = nHeR(298 K)

(1.10 atm)VAr = nArR(298 K)

PNeVNe = nNeR(298 K)

but I can't see how you could calculate the moles of a gas in the mixture (but I do believe the neon can't be found, basically no information for that gas).
 
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  • #2
No one? I really dislike this problem now. :biggrin: :devil:
 
  • #3
the volume of 1 mol of gas is 22.4 L, so you have .123 mol of helium
 
  • #4
Light bulb said:
the volume of 1 mol of gas is 22.4 L, so you have .123 mol of helium

That's volume at STP, it doesn't hold in general.

Question is ambiguous and wording is tricky. The only correct answer is "for none".

--
methods
 
  • #5
Did you translate this out of some other language?
 
  • #6
The book is in English and I typed it exactly as it was written.

Whether or not the pressure and/or volume applied to one specific gas or the whole mixture, I think there's still not enough information...
 
  • #7
Then it's helium ... the gas for which u can compute. It's a gas mixture so if one is at a certain preassure temp, then all are at the same etc...

Basically how many moles are in a 2.75L helium bottle at 1.1atm and 25'C.

PV=nRT be quick! :P
 
  • #8
Lok said:
It's a gas mixture so if one is at a certain preassure temp, then all are at the same etc...

No. If you have a mixture each gas has its own pressure (called partial pressure), and total pressure is sum of partial pressures.

After rereading the question I think 1.1 atm for argon is the partial pressure, so number of moles of argon can be calculated, asssuming given volume and temperature (which seems to be logical). But it is not an ideal gas question, it is a brain teaser.

--
 
  • #9
I agree with your conclusion that argon is the gas that moles can be calculated for. It is an ideal gas question in that the question asks for which gas can moles be calculated using [tex]\newcommand{\chem}[1]{\ensuremath{\mathrm{#1}}}
\chem{n=\frac{PV}{RT}}
[/tex]

It is not straight forward but it is indeed an ideal gas question. Interesting!
 
  • #10
I think what the author had in mind was that the 2.75 L is the volume for all three gases, as well as 25ºC for the gases. Only the argon has a pressure, so the moles of argon only can be calculated.

The funny thing (well, not really) is that the instructor said the moles could be calculated for each gas and each one has the same number of moles...
 

FAQ: Solving Gas Mixture Problem: Calculate Number of Moles

1. How do I calculate the number of moles in a gas mixture?

To calculate the number of moles in a gas mixture, you need to know the total volume of the mixture, the pressure of the mixture, and the temperature of the mixture. You can then use the ideal gas law equation, PV=nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature. Simply rearrange the equation to solve for n and plug in the values to calculate the number of moles.

2. What is the ideal gas law equation?

The ideal gas law equation is PV=nRT, where P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature. This equation describes the relationship between the pressure, volume, temperature, and number of moles of an ideal gas.

3. How do I convert between units when solving gas mixture problems?

To convert between units when solving gas mixture problems, you can use conversion factors. For example, to convert between Celsius and Kelvin temperatures, you can use the equation K = °C + 273.15. For pressure, you can use the conversion factor 1 atm = 101.325 kPa. Make sure to keep track of units and use the appropriate conversion factor in your calculations.

4. Can I use the ideal gas law for any gas mixture?

The ideal gas law is based on the assumption that the gas molecules are small and far apart, and that there are no intermolecular forces between them. This is not always the case for all gas mixtures. If the gas mixture deviates significantly from ideal behavior, the ideal gas law may not give accurate results. In these cases, other equations, such as the van der Waals equation, may be more appropriate.

5. What are some common units for pressure, volume, and temperature when solving gas mixture problems?

Some common units for pressure are atmospheres (atm), kilopascals (kPa), and millimeters of mercury (mmHg). For volume, common units include liters (L) and cubic meters (m^3). Temperature is often measured in degrees Celsius (°C) or Kelvin (K). It is important to make sure that all units are consistent in your calculations to get accurate results.

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