Question

What mass in grams of potassium hydroxide is contained in 500 mL of solution that has a pH of 11.5?

I have a pOH of 2.5

to get [OH]

I do 10^-2.5 to get 0.001581138

Then I divide this by 2 to find the mols in 500 mL

Then, grams = mols x molar mass

= 0.00158``38 x 56.105 g/mol

=0.08 g

Why is my answer wrong?

Answer 1

What mass in grams of potassium hydroxide is contained in 500 mL of solution that has a pH of 11.5?

I have a pOH of 2.5 <b> OK</b>

to get [OH]

I do 10^-2.5 to get 0.001581138
<b>No, (OH^-) = 0.00316 M </b>

Then I divide this by 2 to find the mols in 500 mL
<b> 0.00316/2 = 0.00158 mols in 500 mL.</b>
Then, grams = mols x molar mass

= 0.00158``38 x 56.105 g/mol
<b>I don't know what the ``38 is. Also, you took 0.00158, divided by 2 and still came out with 0.00158? I think you lost track of what you were typing here. 0.00158 mol in 500 is the correct number. </b>
=0.08 g
<b> 0.00158 mols x 56.105 g/mol = 0.08865 g which I would round to 0.0886 g KOH.</b>
Why is my answer wrong? <b>My best guess is that you truncated the number to 0.08 g instead of keeping the other two places of 0.0886 g</b> Check my work.

Answer 2

Wow , thanks you are a really excellent expert!
:)