Gas law

1. Mar 22, 2010

roam

1. The problem statement, all variables and given/known data

A spherical balloon is inflated to a diameter of 36 cm. Assuming that the gas in the balloon is of atmospheric pressure (101.3 kPa) and is at a temperature of 20°C.

(a) How many moles of gas are in the balloon?

(b) If 21% of the molecules in the balloon are oxygen molecules, how many oxygen molecules are in the balloon?

3. The attempt at a solution

(a) I want to use the formula $$PV=nRT$$

Volume is $$v=\frac{4}{3} \pi (0.18)^2 = 0.1357$$

R is the constant 8.314

So $$(101.3)(0.1357)=n8.314(20)$$

Now if I solve for n I get $$n= {13.74}{166.28} = 0.082$$

This is wrong because the correct answer must be 1.02. Do I need to convert the temprature to Kelvin/absolute temprature? ...because I tried that too & it didn't work.

2. Mar 22, 2010

rl.bhat

Given pressure is in kilopascal.
Temperature must be in Kelvin.

Last edited: Mar 22, 2010
3. Mar 22, 2010

mgb_phys

Temperature should be in kelvin

4. Mar 22, 2010

rock.freak667

Your temperature should be in Kelvin (K) and the pressure in Pa

5. Mar 23, 2010

inutard

6. Mar 23, 2010

roam

Okay 20 degrees is 293.15 in Kelvin, and I converted the pressure to pascals. So my equation becomes

$$(101300)(0.1357)=n(8.314)(293.15)$$

n= 5.64

But why is this still not the right answer?

7. Mar 23, 2010

rl.bhat

Check the volume of the gas.
V = 4/3*π*r^3

8. Mar 23, 2010

inutard

= [. I said that earlier. I'm sad now.