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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: mauvery on April 25, 2016, 01:21:10 PM

Title: Find moles of NaOH given initial and final pH.
Post by: mauvery on April 25, 2016, 01:21:10 PM

"How much moles of NaOH is needed to change 500 litres of solution from pH=2 to pH=11?"

Not quite sure how to tackle this question. Would I have to find the moles of hydroxide ions to neutralise the solution first (ie. moles of hydroxide ions needed to reach pH=7) and then from there calculate the amount of hydroxide ions needed to get to pH=11?

Or do I just calculate the difference between the amount of n(OH-) in pH 11 with n(OH-) in pH 11?

 ??? ??? ???

Thank you so much to whoever will reply.

Title: Re: Find moles of NaOH given initial and final pH.
Post by: Borek on April 25, 2016, 02:09:11 PM

You need to take the neutralization into account. At the beginning most of the OH^- added is consumed by the reaction with H⁺.

Title: Re: Find moles of NaOH given initial and final pH.
Post by: AWK on April 25, 2016, 02:29:29 PM

Scale down your problem to 1 liter first, then split it into 2 steps:
1. neutralization from pH=2 to pH=7
2. change pH to 11 by adding more NaOH
Finally scale up to 500 liters.

Title: Re: Find moles of NaOH given initial and final pH.
Post by: mauvery on April 25, 2016, 07:55:50 PM

Quote from: AWK on April 25, 2016, 02:29:29 PM

Scale down your problem to 1 liter first, then split it into 2 steps:
1. neutralization from pH=2 to pH=7
2. change pH to 11 by adding more NaOH
Finally scale up to 500 liters.

Quote from: Borek on April 25, 2016, 02:09:11 PM

You need to take the neutralization into account. At the beginning most of the OH^- added is consumed by the reaction with H⁺.

Thank you so much! Would you also have to take neutralisation into account for this question

"Swimming pool contains 2 million litres of water at pH 7.80 and the pH needs to be changed to pH of 6.80. How many moles of HCl is needed?"

Title: Re: Find moles of NaOH given initial and final pH.
Post by: Borek on April 26, 2016, 02:49:11 AM

Quote from: mauvery on April 25, 2016, 07:55:50 PM

Would you also have to take neutralisation into account for this question

"Swimming pool contains 2 million litres of water at pH 7.80 and the pH needs to be changed to pH of 6.80. How many moles of HCl is needed?"

Yes, it is almost identical.

As a rule of thumb - when you cross the pH=7.0 point you need to take both neutralization (first) and dilution (second) into account (which should be obvious once you start to think what is happening in the solution).