(50)

The coagulating value of an electrolyte is defined as the minimum amount of the electrolyte required to coagulate a unit volume of the colloidal solution in a given time. In this problem, we are given that 20 mL of 0.5 M NaCl solution is required to
coagulate 200 mL of solution in 2 hours.

To find the coagulating value of NaCl, we need to determine the amount of NaCl required to coagulate 1 liter (1000 mL) of the solution in 2 hours. This can be calculated using the following formula:

To find the amount of NaCl required to coagulate 200 mL of the solution in 2 hours, we can use the formula for the number of moles of NaCl used:
where is the concentration of NaCl in moles per liter, and is the volume of NaCl solution used in liters. Substituting the given values, we get:

To find the amount of NaCl required to coagulate 1 L of the solution, we need to scale up the amount of NaCl used by a factor of 5 (since 5 times the given volume of NaCl solution is required to make 1 L of the solution). This gives:

The coagulating value of NaCl can now be calculated as:

Coagulating value

Therefore, the coagulating value of NaCl is 50 (in millimoles per liter).