' A ' is a protecting colloid. The following data is obtained for preventing the coagulation of 10 mL of gold sol to which 1 mL of 10 % NaCl is added. What is the gold number of 'A'? array |c|c|c| Expt. No. & array c Wt (in mg) of A added to gold sol array & Coagulation 1 & 40 & Prevented 2 & 35 & Prevented 3 & 25 & Not prevented 4 & 32 & Not prevented 5 & 33 & Prevented array

Step-by-step solution

Gold number is defined as the minimum amount of lyophilic colloid in milligrams which prevents the flocculation of 10 ml gold sol by the addition of 1 ml of 10 % NaCl solution. More the gold number, lesser will be the protective power of lyophillic colloids. So among the 1,2 and (5), (5) have the lowest amount of ' A ' added to gold sol so the gold number of ' A ' is 33 .