Question

Ou have a 5 ml sample of a protein in 0.5 m nacl. you place the protein/salt sample inside dialysis tubing (see fig. 2-14) and place the bag in a large beaker of distilled water. if your goal is to remove as much nacl from the sample as possible, which would be more effective: (1) placing the dialysis bag in 4 l of distilled wa- ter for 12 h, or (2) placing the bag in 1 l of distilled water for 6 h and then in another 1 l of fresh distilled water for another 6 h

Answer 1

Answer:

Procedure (2)  

Explanation:

Assume the dialyses come to equilibrium in the allotted times.

Procedure (1)

If you are dialyzing 5 mL of sample against 4 L of water, the concentration of NaCl will be decreased by a factor of

$\dfrac{5}{4000} = \dfrac{1}{800}$

Procedure (2)

For the first dialysis, the factor is

$\dfrac{5}{1000} = \dfrac{1}{200}$

After a second dialysis, the original concentration of NaCl will be reduced by a factor of  

$\dfrac{1}{200} \times \dfrac{1}{200} = \dfrac{1}{40000}$

Procedure (2) is more efficient by a factor of  

$\dfrac{40000}{800} = \mathbf{50}$