Exp. 10: Molar Mass of Oxygen

You will determine the molar mass of oxygen experimentally and compare your value with the known molecular mass of oxygen.

KClO₄(s) --> KCl(s) + 2 O₂(g)

You will heat solid potassium perchlorate (KClO₄) and generate oxygen.  The mixture will also contain a catalyst to make the reaction run more easily.  The catalyst is manganese dioxide (MnO₂) and is not consumed in the reaction.  So, there is no net change in the catalyst and it is not shown in the balanced equation above.   What general type of reaction is this?

This laboratory exercise will introduce you to some of the physical aspects of gases.   Gases are often described in terms of properties such as volume, pressure, temperature and quantity.

These properties are all interrelated and can be summarized by the Ideal Gas Law:

P·V = n·R·T

In this relationship, P is the pressure, V is the volume, T is the temperature and n is the number of moles of a gas (quantity).  The term, R is a proportionality constant known as the gas constant.  The gas constant can have many numerical values depending on the units it is expressed in:

0.082057 L atm/mole K;

8.3145 J/K mole;

8.3145x10¯³ kJ/K mole;

1.9872 cal/K mole

We can use the Ideal Gas Law to evaluate the relationships between various physical aspects of gases.  For example, what is the relationship between pressure and volume?

P·V = n·R·T

P = n·R·T/V     

or   P (is proportional to) 1/V

So, pressure and volume are inversely (reciprocal) proportional.  You can figure that from the Ideal Gas Law.

What is the relationship between pressure and temperature?

P·V = n·R·T

or   P (is proportional to) T

So, pressure and temperature are directly proportional. 

Another way of using the Ideal Gas Law is to substitute in an equivalent expression for the number of moles, n.     The number of moles of any substance is the number of grams of that substance divided by the molar mass (MM);

n = g/MM

[g/(g/mole) = mole]

so, we can substitute this into the Ideal Gas Law to get:

PV = (g/MM)RT

If we rearrange to solve for molar mass, we get:

MM = g·R·T/P·V 

Therefore, if we know the mass, the pressure, the volume and temperature of a gas, then we can calculate its molar mass.  

You will determine the molar mass of oxygen experimentally and compare your value to the expected value. 

(Molar mass O₂ = (2 x 15.9994 g/mole) = 31.999  g O₂/mole

Set-up for generating oxygen.  KClO₄ and MnO₂ are placed in the Erlenmeyer flask on the left.  This mixture is heated and the O₂ generated will displace the water from the Florence flask in the middle.  The water will collect in the beaker on the right as it is displaced.

 

Sample Data:

Mass of flask + KClO₄ + MnO₂ (initally)                   27.8539 g

Mass of flask after heating                                         27.4847 g

Volume of O₂ gas (H₂O displaced)                             367 mL

Temperature of O₂ gas (temp. H₂O)                             19.4 °C

Atmospheric pressure (mm of Hg)                                 631.8

Pressure of H₂O vapor (mm of Hg)                                 13.885

Calculations:

Mass of O₂ generated                                           0.3692 g

Volume O₂ in liters                                                0.367 L

Temperature of O₂ in Kelvin                          292.5 K

Pressure O₂ gas in mm                                  617.9 mm Hg

[P(O₂) = P(atm) - P(H₂O)]

                    [631.8 mm Hg  -   13.885 mm Hg  =  617.9 mm Hg]

Pressure O₂ gas in atm.                             0.8130 atm

                   [617.9 mm Hg x (1 atm/760 mm Hg) = 0.8130 atm]

Molar mass of O₂ calculated                               29.7 g/mole

MM = g·R·T/P·V

(0.3692 g)x(0.082057 L atm/mole K)x(292.5 K)      =        29.7 g/mole

                       (0.8130 atm)  x  (0.367 L)

% Error                                                               -7.18%

((your value - actual value)/actual value) x 100% = percent yield

[(29.7 - 31.999)/31.999)x100% = - 7.18 %

What does the negative sign indicate?

This is the last lab of the semester:

Sorry about that, you'll just have to deal with it!! 
This write-up will be completed and turned in before you leave the lab.

Notebook pick-up.

Tuesday & Wednesday labs (sections 13 thru 16):  Pick up notebooks, Tuesday, 11/23/04 in CBB 001 (lab) after 9 AM.

Thursday through Monday labs (sections 11, 12, 17 thru 20):  Pick up notebooks, starting Monday, 11/29/04 in CBB 001 (lab) after 9 AM

Remember the second exam on Nov. 30 and Dec. 2!!

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