# More gas law problems

1. Oct 13, 2008

### blackjack18

what volume is occupied by 2.0 g of He at 25 degrees C and a pressure of 775 mm?

2. Oct 13, 2008

### Rake-MC

$$Pv = nRT$$

P = pressure
v = volume
n = number of moles
R = universal gas constant
T = temperature.

Calculate how many moles are in 2.0 grams of helium, divide both sides of the equation by P.

Solve.

EDIT: Remember to have your units correct, IE Kelvin, kilograms etc

3. Oct 13, 2008

### blackjack18

what is the constant? or is there one? if so, how do i determine what it is?

4. Oct 13, 2008

### Rake-MC

R is the universal gas constant. It is always the same. In si units, it is: 8.314472 Joules per mol kelvin.

5. Oct 13, 2008

### blackjack18

ok, im not sure about this, but is the answer 1.59?

6. Oct 13, 2008

### Rake-MC

What did you use as your numbers?

7. Oct 13, 2008

### blackjack18

775=0.5(constant)(298), then divided by 775

8. Oct 13, 2008

### Rake-MC

That equation is: $$p = nRT$$
You need volume on the left hand side to solve for it.
Also, you wrote in the question description that pressure = 775mm, I am going to assume that this is mmHg.
You are going to need to convert this to pascals.

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