Here are some student data from the experiment where you placed BaCl2 and Na2SO4

Question

Here are some student data from the experiment where you placed BaCl2 and Na2SO4 in different wells within an agar plate. Ba++ and SO4-- came together to form a precipitate.

Distance traveled (mm)    Time (min)

Ba++          SO4--   

4.5        5.5                    9   

4.5        5.0                    11

4.0        5.0                    11

5.0        6.0                    20   

Mean     4.5        5.38                  12.7

Distance traveled (mm)    Time (min)

Ba++       SO4--

9.0       11.0                 73

11.0      9.0                    86

9.0       11.0                 85

8.0       12.0                 87

Mean: 9.25      10.75                82.7

Why did sulfate travel farther than barium before they joined to form the precipitate?

Calculate the ratio of distances traveled that you would expect from first principles, and then compare this with the average of the ratios observed.

Explanation / Answer

Sulfate (Mr ~ 96) is lighter and hence, diffuses faster than Barium (Mr ~ 137).

For calculating the ratio of distances traveled, we use the Graham's law*, which states that the rate of diffusion is inversely proportional to the square root of its mass.

Thus,

(distance traveled by barium/ distance traveled by sulfate) = (molecular mass of sulfate/ molecular mass of barium) = (96/137) = 0.837

Average ratio observed = (9.25/10.75) = 0.860

Thus, the expected and observed ratios are quite close to each other.

(*You may note that Graham's law pertains to gaseous diffusion; some deviations are expected while considering the diffusion of solutes through a semi-solid medium)

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