Hydroxide in a acidic solution

Question

A  M solution of HBr is made. 

What is the pH of this solution?

Correct

What is the pOH of this solution?

Correct

 

What is the [OH^-] in the solution?  Record this answer in scientific notation using 3 sig figs.

the pH is 1.09 the pOH is 12.91

Answer 1

Hello,

So recall the formula that pH = -log[H+], meaning the concentration of the acid. To figure out the pH, we would calculate -log(8.18 *10^-2 M) , which = 1.087, or 1.09.

To calculate pOH, recall that pH + pOH = 14, so since we know that pH = 1.09 (computed from the previous part), we can plug it in to the equation, yielding 1.09 + pOH = 14. Solve for pOH: 14 - 1.09 = 12.91.

Another way we can think about it is, for any chemical, the concentration of hydronium (H+) multiplied by the concentration of hydroxide (OH-) must = 1.0 * 10^-14. The formula would be: [H+][OH-] = 1.0 * 10^-14. We know that the concentration of H+ is 8.18 * 10^-2, and we can solve for the concentration of OH- with: (8.18*10^-2)([OH-]) = 1.0 * 10^-14. We solve for [OH-], which gives us 1.222 * 10^-13. Then, we can use the same method we calculated for pH, (-log[H+]) and solve for pOH, by doing -log[OH-]. -log[1.222*10^-13) = 12.91 = pOH