Question

pH is a measure of proton activity and not concentration.

(a) Calculate the pH of water (T = 25o C) containing only 10-7 M H+ , i.e. in ultrapure water.

(b) If the same water also contains 0.36 M NaCl, re-calculate the pH. Use the Debye-Huckel equation with A = 0.511 as an approximation to calculate the activity coefficient.

(c) Explain the difference in pH.

Answer 1

Solution:-

A) pH of water (T = 25o C) containing only 10^-7M H+

pH = -log₁₀[H⁺]
pH = -log₁₀( 10^-7)
pH = 7

B) If the same water also contains 0.36 M NaCl, re-calculation of the pH by Using the Debye-Huckel equation with A = 0.511 as an approximation to calculate the activity coefficient.

the activity coefficient is calculated as

logγ=−0.51(z2)(√u)/{1+(α(u√305)}

logγH+=−.51(1²)()/{1+( 0.511(0.36√305) }  

logγH+= 0.07263709944

γH+ = 1

pH=−logaH+=−log(γH+⋅1.0⋅10⁻⁷)

pH=−logaH+=−log(1⋅1.0⋅10⁻⁷)

pH = 7

(c) The difference in pH by the use of debye -huckel equation is that PH depends upon activity coefficient so Ionic strength is a measure of the total ion concentration in solution, however, species with higher charge can also have elctrostatic interactions with other species. Thus we have to analyse ionic strength and then further ph is depend on these criteria this creates difference in PH