A dilute solution of HBr is found to have a pH of 3.55. What is [H₃O⁺]?

Question

Quizplus · Accepted Answer

From the pH of the solution, we can find the concentration of H+ ions using the equation pH = -log[H+]. 
So, -3.55 = log[H+]. 
Taking anti-log on both sides, we get [H+] = 3.55×10^-4 M. 
Now, HBr is a strong acid and it will completely dissociate in water, so [H+] = [Br-]. 
Thus, [HSBrO] = [HBr] - [H+] = [Br-] = 3.55×10^-4 M. 
Kb for HSBrO (the conjugate base of HBr) can be calculated using the equation Kw = Ka x Kb, where Ka is the dissociation constant of HBr (which is very large for a strong acid and can be considered infinite). 
Thus, Kb = Kw/Ka = 1.0 x 10^-14/∞ = 0. 
Finally, we can use the equation for Kb to find [HSBrO]: Kb = [H+][Br-]/[HSBrO]. 
Plugging in the values we have, we get: 
0 = (3.55×10^-4)^2/[HSBrO]. 
Thus, [HSBrO] = 2.8×10^-4 M. 
But we need to find [HS+BrO-], which is the same as [HSBrO]. 
Therefore, the answer is choice C, 2.8×10^-4 M.

A Dilute Solution of HBr Is Found to Have a PH