What is the formula for calculating the mass of air in a piston?

In summary, the conversation discusses a problem with using the given formula for calculating mass in a thermodynamics question. The expert suggests checking for errors, including units, and using the correct formula for part (b). After further discussion and calculations, the correct answer is determined to be 0.515 kg.
  • #1
Gregs6799
35
0

Homework Statement


upload_2018-11-25_19-18-57.png


Homework Equations


pV = (m/Mr)RT (this is the formula given but I got the wrong answer using it)

The Attempt at a Solution


Mass = PV / RT
R = 8.314
P1 x V2 / R x T1
M = 220 x 0.1 / 8.314 x 298 = 788.549kg
Mass = 788.549kg

P = 440000
V = 0.1
R = 8.314
T = 298
440000 x 0.1 = 44000
M = mRT / PV
M = 44000 x 8.314 x 298 / 440000 x 0.1 = 24.77g.
Mass of air in piston = 24.77g.
P1 = 440000

V2 = 0.1
R = 8.314
T = 25 + 273 = 298 (from Celsius to kelvin)
P1 x V2 / R x T1
M = 440000 x 0.1 = 44,000
8.314 x 298 = 2477.572
44000 / 2477.572 = 17.759kg

All the above values are correct but the formulas and answers are incorrect.
I've posted here before and your probably sick and tired of trying to help me out but if anyone can give me the correct formula for question b only a is correct just b, I'd be very appreciative, but just the formula, try not to go too in depth with any replies as it will go over my head and confuse me all I need is the formula and in what order do the values need to go in, he answer should be in kgs.

Thank you.[/B]
 

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  • #2
Gregs6799 said:

Homework Equations


pV = (m/Mr)RT (this is the formula given but I got the wrong answer using it)
Hello.
Note the Mr in the formula. Did you include that in your calculation for part (b)?
You would be able to catch such errors if you included units when doing the calculation.
 
  • #3
I think Mr = 44000 / 17.75 = 2478 and probably didn't put in the formula, not sure what units that would be in.
 
  • #4
Gregs6799 said:
I think Mr = 44000 / 17.75 = 2478 and probably didn't put in the formula, not sure what units that would be in.
When you are sure of the correct units, please let us know. We will be waiting.
 
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Likes gneill
  • #5
I tried replacing (m/Mr) with n (moles) nRT
17.75 x 8.314 x 298 = 43976 (unless that's in grams is way too high.)
And RT / PMr
8.314 x 298 / 44000 x 2478 = 139.53, again too high if its in kgs.
 
  • #6
$$moles=\frac{PV}{RT}=\frac{(220000)(0.2)}{(8.314)(298)}=\frac{(440000)(0.1)}{(8.314)(298)}=17.76\ moles$$Molecular weight of air = 29 gm/mole. So, $$mass=(17.76)(29)=515\ grams=0.515\ kg$$
 
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Likes Delta2
  • #7
So I wasn't far off the right answer with
440000 x 0.1 = 44,000
8.314 x 298 = 2477.572
44000 / 2477.572 = 17.759 moles

just didn't multiply by 29, which I didn't see in the given formula, unbelievable.

Sorry for taking up much of your time, your either born with the brains or your not.
Many thanks.
 

Related to What is the formula for calculating the mass of air in a piston?

What is the mass of air in the piston?

The mass of air in the piston is the total amount of air that is contained within the piston. This can vary depending on the size and volume of the piston, as well as the density of the air inside.

How is the mass of air in the piston calculated?

The mass of air in the piston can be calculated by multiplying the density of air by the volume of the piston. The density of air can be found using the ideal gas law, which takes into account the temperature and pressure of the air.

Why is the mass of air in the piston important?

The mass of air in the piston is important because it affects the overall performance and efficiency of the piston. It also plays a crucial role in the combustion process, as the amount of air present can impact the fuel-to-air ratio and the power output of the piston.

How does the mass of air in the piston change during operation?

The mass of air in the piston can change during operation due to factors such as temperature, pressure, and air intake. As the piston moves and compresses the air, the mass will decrease due to a decrease in volume and an increase in density.

Can the mass of air in the piston be altered?

Yes, the mass of air in the piston can be altered by adjusting the air intake or by changing the temperature or pressure of the air. This can be done through various methods such as using a turbocharger or a supercharger, or by changing the size of the piston or the air intake system.

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