Question

3. Explain the series of color changes that can occur on thymol blue indicator by the addition of the following solutions. In each case, give the approximate pH of the solution Color change Red to Yellow Yellow to Blue pH range 12 to 2.8 Thymol blue 8.2 to 9.1 a) A few drops of thymol blue are added to 0.1 M aqueous solution of HCl(aq). b) By adding some solid sodium acetate, NaCH,O2 to a solution of (a) e) By adding only a few drops of sodium hydroxide solution, NaOH to (e) d) On adding a larger quantity of sodium hydroxide solution to the sclon is 4. Cobait () sat can exist as [CoCl and [CoH O lons in aqueous solutions. The following equilibrium reaction represents the two types of complexes of cobait with two different colors CopMO (aq. pink) 6 HO) es- (CoCL (aq. blue) 4 Cr(ag) a) What change in color do you expect when you add 10 drops of 12 M HCI to an aqueous solution of cobalt salt? Explain your answer in terms of LeChatier principle. b) To the above mixture, if you add distilled water, HO drop-wise, what changes you can expect to occur? 3

Answer 1

ans 3

A) O.1M HCl have H+ conc. is = 0.1M

pH = -log[ H+] =-log0.1= 1

when few drop of thymol blue add color of solution is almost red

B) Now few drop of Na2C2H2O2 is added to solution in (A)

then Na2C2H2O2 act as base it take two H+ and form C2H2O2H2 means H+ concentration is decreases so the pH of solution A is increases slightly to almost 2.5

now the colour of solution is change to yellow.

C) Few drops of NaOH is added to solution (B) , now NaOH is strong base it changes pH by few drops. Now the pH shift almost 3.2 the color of solution still remain yellow

D) Now in solution C large amount of NaOH is added it increases pH almost 8.9 now the color of solution is Blue. The pH is increasing due to NaOH is taking H+ .i.e. neutralization occurring.

ans 4

[ CoCl4]2-(blue) + 6H20 = [ Co(H20)6]2+(pink)    +    4Cl-

A) 10 drops of 12M HCl added is added in above reaction means 10 drops of 12M Cl-. So according to the Le Chatelier's principle say that when Cl- is added in the above reaction system response reverse to changing factor Cl- means the reaction is goes toward left side to minimize free Cl- to forming again [ CoCl4]2-. So the equilibrium is shift toward left and know the concentration of [ CoCl4]2- is increases and the color of the solution is Blue.

B) Now you added distilled water know your reaction mixture shift toward right side and the solution color is pink . It same explain above that is Le Chatelier's principle say that when H2O is added in the above reaction system response reverse to changing factor H2O means the reaction is goes toward right side to minimize H2O to forming again [ Co(H20)6]2+. So the equilibrium is shift toward right and know the concentration of [ Co(H20)6]2+ is increases and the color of the solution is again turn to pink.