Question

What is the pH difference of two samples if the concentration of [H+] ions is 1000-fold less in the second sample? A. The pH of the first sample would be 3 pH units lower than the second sample. B. The pH of the first sample would be 2 pH units higher than the second sample. C. The pH of the first sample would be 2 pH units lower than the second sample. D. The pH of the first sample would be 3 pH units higher than the second sample. If the pH of a nitric acid solution is 2.34, then what is the molarity of nitric acid in the solution?

Answer 1

Ans. #1. Let, [H⁺] in sample 1 = 0.001 M

            Now, pH of sample 1 = -log [H⁺] = -log (0.001) = 3.0

# Given, [H⁺] in sample 2 = [H⁺] in sample 1 / 1000 = 0.001 M / 1000 = 0.000001 M

Now,  

            pH of sample 2 = -log (0.000001) = 6.0

# So, correct option is- A. The pH of first sample (sample 1) would be 3 units lower than the second sample (sample 2).

Note that pH is –ve log. So, lower is the [H⁺], higher would be the pH.

#2. Given, pH = 2.34

Now,

            [H⁺] = antilog (-pH) = antilog (- 2.34) = 0.00457

Hence, [H⁺] in nitric acid solution. 0.00457 M

# Being strong acid, 1 mol HNO₃ donates 1 mol H⁺ to the solution. So, the [HNO₃] in solution must be equal to its [H⁺].

Therefore,

            [HNO₃⁺] = 0.00457 M = 4.57 x 10^-3 M