Calculating Grams of Helium Needed for Blimp to Rise

[Myung]

Homework Statement

Blimps are being considered for use as freight carriers. A scale model rises when filled with helium to a volume of 55.0 dm3. When 1.10 mol He is added to the blimp, the volume is 26.2 dm3. How many more grams of He must be added to make it rise? Assume constant T and P. (4.003 g He = 1 mol He)

Homework Equations

PV = nRT
R = 0.8206 ( litres-atm/mole-K )
1 dm^3 = 1L

The Attempt at a Solution

Required volume to fill is 28.8 dm^3 or L

PV = nRT

Assume T and P is constant?
How do we do that? That is my question.

 

[Andrew Mason]

Homework Statement

Blimps are being considered for use as freight carriers. A scale model rises when filled with helium to a volume of 55.0 dm3. When 1.10 mol He is added to the blimp, the volume is 26.2 dm3. How many more grams of He must be added to make it rise? Assume constant T and P. (4.003 g He = 1 mol He)

Homework Equations

PV = nRT
R = 0.8206 ( litres-atm/mole-K )
1 dm^3 = 1L

You are trying to find n. You want to express n as a function of V. If P and T are constant, what is the relationship? How would you find P/RT (which is constant) from the information provided?

AM

 

[Myung]

You are trying to find n. You want to express n as a function of V. If P and T are constant, what is the relationship? How would you find P/RT (which is constant) from the information provided?

AM

They are inversely proportional to each other ( P and T ) ,

Universal Gas Constant = 8.206 ( Litres - atm / mole - K )

n = PV/RT

1 atm is the constant Pressure at sea level but I don't know the temperature?

 

[Myung]

bump!

 

[gneill]

They are inversely proportional to each other ( P and T ) ,

Universal Gas Constant = 8.206 ( Litres - atm / mole - K )

n = PV/RT

1 atm is the constant Pressure at sea level but I don't know the temperature?

You don't need to know the pressure or temperature; It's enough to know that they are constants.

In your expression for n above, group all the constants together and replace them by a single new constant. Let's call it k. Now, in the problem statement you're given a particular case for n and V. Find k from that particular case. You can then use this k value to proceed.

 

[Myung]

You don't need to know the pressure or temperature; It's enough to know that they are constants.

In your expression for n above, group all the constants together and replace them by a single new constant. Let's call it k. Now, in the problem statement you're given a particular case for n and V. Find k from that particular case. You can then use this k value to proceed.

P/T = nR/V

Let P/T = k

k = nR/V

Given the case that there are 1.10 moles in 26.2 dm^3/L of He

k = [1.10moles ( 0.8206 ) L-Atm/moles-K] / 26.2L of He

k = 0.03445267176 Atm/Kelvin

Constant value gained!

n = V/R * (k)

n = 28.8 L / 0.8206 Litres-atm/mole-kelvin * ( 0.03445267176 Atm/Kelvin )

n = 1.21 moles

1 mole = 4.003 g HE

He = 4.84 Grams.

THANKS!