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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: SLC91 on July 02, 2018, 01:31:01 AM

Title: Finding pH from mass of OH- ions
Post by: SLC91 on July 02, 2018, 01:31:01 AM

Hi all,
I have a problem for which I was given the solution but not the way it was worked out, and my answers don't match.

The problem in question:

Calculate the pH of a solution that contains 0.00017g OH- per 100 mL.
The solution should be 9.

The way I am trying to work it out (please help me figure out where I went wrong)

Step 1: Convert g per 100mL into g/L:
0.00017 g per 100 mL = 0.0017 g/L

Step 2: convert g/L of OH- into m/L
mM of OH = 17
mass = 0.0017g
Therefore: 0.0017/17 = 0.0001

Step 3: Convert into pOH
-log₁₀(0.0001) = 4

Step 4: Convert pOH into pH
pH + pOH = 14.
14-4= 10.

My [OH^-] should be 0.00001 mol/L, but I'm ending up with 0.0001 instead. Are my conversions wrong, perhaps?

Thanks for the help.

Title: Re: Finding pH from mass of OH- ions
Post by: mjc123 on July 02, 2018, 04:20:46 AM

Your working looks correct to me. I think the book has got it wrong. They are assuming [OH^-] = 10^-5M, but they have 10^-5 moles in 100 mL.
Even textbooks and teachers get it wrong sometimes.

Title: Re: Finding pH from mass of OH- ions
Post by: SLC91 on July 03, 2018, 09:48:20 PM

Quote from: mjc123 on July 02, 2018, 04:20:46 AM

Your working looks correct to me. I think the book has got it wrong. They are assuming [OH^-] = 10^-5M, but they have 10^-5 moles in 100 mL.
Even textbooks and teachers get it wrong sometimes.

That's what I thought. Thanks! :)