# Physics 212, Quiz #5b: Jan 15, 1998

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Argon gas contains molecules, each of which is a single argon atom. The mass of a argon atom is 40 atomic mass units.

Question 1: What is the mass of 1 mole of argon gas?

```  Answer: mass (grams) = mass of molecule (atomic mass units)

= 40 g

```

Joe places 8 grams of argon gas into a balloon.

Question 2: How many moles of argon are in the balloon?

```  Answer: # moles = (mass of sample)/(mass of 1 mole)

= (8 g)/(40 g) = 0.2 moles
```

Question 3: How many molecules of argon are in the balloon?

```  Answer: # molecules = (# of moles)*(Avagadro's number)

= (0.2) * (6.022 x 10^23) = 1.20 x 10^23

```

The balloon inflates to a diameter of 12 cm.

Question 4: What is the volume of the balloon, in cubic meters?

```  Answer:            4*pi
3

4*pi*(0.06 m)^3
= --------------- = 9.05 x 10^(-4) m^3
3

```

The gas inside the balloon has a temperature of 15 degrees Celsius.

Question 5: What is the pressure of the gas inside the balloon?

```  Answer:  PV   =  nRT

Note that temperature must be expressed in Kelvin:

T(kelvin) = T(celsius) + 273 = 288 K

Solve for pressure P:

nRT     (0.2 moles)*(8.31 J/mol*K)*(288 K)
P = ------- = ----------------------------------
V             9.05 x 10^(-4) m^3

= 5.29 x 10^5 N/m^2 = 5.29 x 10^5 Pa

```