April 03, 2020, 07:51:10 PM
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Topic: How will each of these additions change the pH of the solution?  (Read 175 times)

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Offline anastasiaaleexev

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1. Adding NaNO3 to a HNO3 solution
2. Adding NaCN to a HCN solution
3. Adding NaOH to a NH3 solution

I said that all 3 will increase in pH. Is that correct?

Offline chenbeier

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Re: How will each of these additions change the pH of the solution?
« Reply #1 on: March 18, 2020, 02:15:28 AM »
No.1 I would think again. No.2 is also questionable, regarding built up of a buffer.
No.3 is correct.

Offline anastasiaaleexev

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Re: How will each of these additions change the pH of the solution?
« Reply #2 on: March 18, 2020, 02:49:52 AM »
My thinking was that when you add NaNO3 and the NaCN, you will be increasing the concentration of the products (NO3- and CN-), so the equilibrium will shift to the reactants, causing [H+] to decrease.

Since this is incorrect, what would be the explanation for it?

Offline Borek

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Re: How will each of these additions change the pH of the solution?
« Reply #3 on: March 18, 2020, 03:59:54 AM »
A lot depends on how much detailed answer you are seeking, you asked in a High School forum, so I assume HS level.

In the case of a strong acid adding salt doesn't matter at all - it is still fully dissociated and (assuming there were no volume/concentration changes) pH doesn't change. In reality it is a bit more complicated.

You are right with 2. though, yes, presence of CN- will shift the dissociation and the pH will go up.

3. is OK as well.
ChemBuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

Offline anastasiaaleexev

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Re: How will each of these additions change the pH of the solution?
« Reply #4 on: March 18, 2020, 07:39:34 AM »
Got it, thank you! :)

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