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Graham's Law of Diffusion Lab

Mr. McLeod, use 1 foot glass cylinders, they dry quicker but still work Theory

Graham's Law of Diffusion deals with the rate of diffusion/effusion of gases (ri). It states that the ratio of the rate of diffusion of two gases is inversely proportional to the ratio of the square root of the gas' molar mass (molecular weight). The law assumes a given number of particles of each gas (1 mole of each gases).

The rate of diffusion is defined as the distance the gas particle travels in a given time. So if you release the gases at the same time, the one with the higher rate of diffusion will have traveled further or ri = di where d is the distance in cm (or other length units)

In this lab, we will look at the gases, ammonia (NH3), and hydrogen chloride (HCl). The HCl looks like hydrochloric acid. It is but since acids only exist in the aqueous phase, then we call this compound another name in the gas phase (hydrogen chloride). Hydrochloric acid is actually generated from bubbling this compound through water. So, to generate hydrogen chloride, we will simply expose concentrated hydrochloric acid to the air. The concentrated acid is located in the exhaust hood. DO NOT TAKE THE CONTAINER OUT OF THE HOOD.

The ammonia, NH3, will be generated from aqueous solution ammonium hydroxide, NH4OH, that decomposes into ammonia and water in the following equation, 1 NH4 --> 1 H2O + 1 NH3. The concentrated ammonium hydroxide solution will be located in the window sill (back wall window nearest the fan).

These two gases will react together to form a white solid, ammonium chloride (NH4Cl). We will use this fact to determine how far, in cm, each gas has traveled over a given time. The distance traveled by each gas is the difference between the starting location (location of Q-tip) and the location of the white ring. From this data, we can then determine the relative ratio of rate of diffusion of the gases. We can compare this experimental ratio to the ratio determine from the inverse square root of the molar masses of the two gases. If Graham's Law is correct, these two ratio should approximately the same values.


Q-tips are located next to each chemical

1. Take a glass tube and a meter stick (meter side next to tube) and put it at your lab station. Label each end of tube via small pieces of paper using wax pencils.
2. One learner will dip a Q-tip into the concentrated ammonium hydroxide solution and place it next to one end of glass tube.
3. The other learner will do the same with the hydrochloric acid.
4. At the same time, put each Q-tip into the appropriate end of the glass tube. One half of the Q-tip should be in the glass tube (each Q-tip should be approximately the same distance into the tube).
5. Let the gases diffuse until you see a small white ring in the glass tube.
6. With the meter stick determine the distance each gas traveled (in cm). starting at Q-tip and ending at white ring.
7. Record your distance in the data table.
8. Each partner takes a Q-tip and run it under water (all HCl in lab bench faucets and others in back sink faucets)
9. Run dry air (via the hair dryer) through the glass tube to get rid of the white ammonium chloride and any liquid drops inside the tube.

Observation / Data


1. Ratio of distances gas travels (indicate each gas):

2. Molar mass of NH3:

3. Molar mass of HCl:

4. Ratio of molar masses (indicate each gas):


1. Write down the ratio of the molar masses and of the distance traveled of each gas next to each other separated by an equal sign. Is Graham's Law supported? Explain.

2. What concept is the basis of Graham's Law of diffusion? (Hint: look in your notes on how we got the equation for the law). Explain using kinetic molecular theory (KMT)

3. What is the "street" definition of Graham's Law (simple way of remembering the law):

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Last edited June 8, 2005 10:26 am (diff)