Question

The conjugate acid-base pair pf imidazole hydrochloride and imidazole can be used to make buffers with a pH of 6.5 to 7.5. Often however, buffers are prepared starting from only one of the two species in the pair and a strong acid or base. Ignore activities for all calculations in this problem.

a) Calculate how many milliliters of 1.000 M HCl should be added to how many grams of imidazole to prepare 250.0 mL of 0.150 M buffer at pH 7.100.

b) If you mixed exactly what you calculated in part (a), the pH would NOT be 7.100. Why is that? Describe how you should actually prepare tihs buffer in the lab at the correct pH. Give step-by-step instructions.

c) i. Find the pH if 1.00 mL, 10.0 mL, and 50.0 mL of 1.00 M HCl is added to the buffer described in part (a).

ii. Now find the pH if you added those same volumes of HCl above to an unbuffered solution of the same volume.

Answer 1

a) As we know the formula v_{1 *} m₁₌v_{2 *} m_{2, - - - - - - - - - -(i)}

where v and m are the volume and molarity of the solution respectively,

v₁ represents the volume of solution of 1.000 M of HCl,

v₂ represents the volume of solution of 0.150 M of HCl (250ml given)

m₁ represents the molality of 1.000 M of HCl (which is 1.000)

m₂ represents the molality of 0.150 M of HCl ( which is 0.150)

Putiing these values in equation (i)

v₁ * (1.000) = 250 * (0.150)

v₁ = {250 * (0.150)/(1.000)}

v₁ = 37.5 - - - - - (ii)

which is the volume required of 1.000 M of HCl to make solution to 0.150 M of HCl

Now - - - - (iii)

- - - - -(iv) - - - (v)

- - - - - (vi)

Putiing (iv), (v), (vi) in (iii), we get

0.150=weight of solute(w)/(68*0.250)

w=68*0.250*0.150

w=2.55 g

This means 2.55g of imidazole should be added to 37.5ml of 1M of HCl to make 250ml buffer of 0.150M.

b) After mixing the two solutions the pH would be less than 7.100 because of the present of H⁺ ions, the H⁺ ions reduces the pH to less than 7.100.

To prepare the buffer solution of 7.100, take the following steps

-> Take 500 ml of water in the container

-> Now add 68g of imidazole to it

-> Then add 36.5ml of HCl solution to it

-> Adjust the solution to pH 7.100 with the pH meter

-> Add distilled water to make the volume upto 1000ml

c) i) when 1.00 ml, 10.0 ml , and 50.0 ml of 1.00 M of HCl is added to the buffer, the pH of the solution remain almost nearby to 7.100 because in buffer solutions are those solutions in which the pH does not vary by much, therefore by adding different volumes of HCl the pH would remain as close to 7.100.

ii) When 1.00 ml , 10.0 ml and 50.0 ml of 1 .00M of HCl is added to the unbuffered solution the pH decreases due to increase in the concentration of H⁺ ions which is the cause of change in pH ,For addition of 50.0 ml of 1.00 M of HCl to unbuffered solution the pH should be 2. For addition of 10.0ml of 1.00M of HCl to unbuffered solution the pH should be 4. For addition of 1.00ml of 1.00M of HCl to unbuffered solution the pH should be 5.7.