Question

Calculate the required volumes of 0.1 M Tris and 0.1 M Tris HCl to prepare three 100 ml solutions at pH 9.00, 8.08, and 6.60.

Answer 1

The henderson-hasselbalch equation is p^H = p^{K _a}+ log ([base]/[acid])

Tris is the base and Tris HCl is acid

p^K_a of Tris is 8.07 at 25 ^oC

For p^{H =} 9.00

log ([base]/[acid]) = p^H - p^K_a

= 9-8.07

= 0.93

[base]/[acid] = 8.511

[buffer] = [Base] + [Acid]

Volume of Base + Volume of Acid = 100 ml

Volume of Acid (V_a)= 100 - Volume of Base (V_b)

Concentration of Base and Acid are equal then [Base]/[Acid] = Volume Base/Volume of acid = 8.511

V_b/100-V_b = 8.511

9.511V_b = 851.1

V_b = 89.48

V_a = 100-89.48 = 10.52

volumes of 0.1 M Tris = 89.48 ml

volumes of 0.1 M Tris HCl = 10.52 ml

For p^H = 8.08

volumes of 0.1 M Tris = 50.56 ml

volumes of 0.1 M Tris HCl = 49.44 ml

For p^H = 6.60

volumes of 0.1 M Tris = 3.29 ml

volumes of 0.1 M Tris HCl = 96.71 ml