Finding PH

Mar 2020
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I think number 1 and 2 are correct? Are there 2 answers?
 
Apr 2015
133
94
This is where you must put your chemist's (or pharmacist's) hat on and look at your answer to see if it makes sense.

I think number 1 and 2 are correct?
for (1) you are asked for the pH of very dilute hydrochloric acid.

The pH must be less than 7 so how can 8 be correct?

Yes I know this is what you will get if you just dilute the solution 1000 times and bung 0.00000001 into your calculator and take the negative of the log.

But this is a very dilute solution , comparable with the concentration of hydrogen ions in pure water.

Have your studies covered how to handle this situation?

The actual pH works out at 6.98

Some online pH calculators will give you pH = 5 for number (4).
Actually it works out to 4.73 since sulphuric acid is diprotic, but Ka for the second proton is low.
 
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Mar 2020
25
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This is where you must put your chemist's (or pharmacist's) hat on and look at your answer to see if it makes sense.



for (1) you are asked for the pH of very dilute hydrochloric acid.

The pH must be less than 7 so how can 8 be correct?

Yes I know this is what you will get if you just dilute the solution 1000 times and bung 0.00000001 into your calculator and take the negative of the log.

But this is a very dilute solution , comparable with the concentration of hydrogen ions in pure water.

Have your studies covered how to handle this situation?

The actual pH works out at 6.98

Some online pH calculators will give you pH = 5 for number (4).
Actually it works out to 4.73 since sulphuric acid is diprotic, but Ka for the second proton is low.
I study HH equation. So, i am supposed to be able to find pH of them.
In (1) There are $10^{-7} mol $ of HCl ?
 
Apr 2015
133
94
Check question (1) carefully.

Yes I know this is what you will get if you just dilute the solution 1000 times and bung 0.00000001 into your calculator and take the negative of the log.
What is 0.00000001 ?
 
Apr 2015
133
94
That's better.

:)

Concentration [HCl] = Concentration [H+] = [Cl-] = 1 x 10^−8M



But in pure water

[H+] = [OH-] = 1 x 10^−7M

So what is the total concentration of [H+] in the solution ?

What therefore is the pH of the solution?

Remember I said this is a very dilute solution

For less dilute (more concentrated) solutions we can ignore the effect of the water solvent.
 
  • Like
Reactions: Helly
Mar 2020
25
0
.
That's better.

:)

Concentration [HCl] = Concentration [H+] = [Cl-] = 1 x 10^−8M



But in pure water

[H+] = [OH-] = 1 x 10^−7M

So what is the total concentration of [H+] in the solution ?

What therefore is the pH of the solution?

Remember I said this is a very dilute solution

For less dilute (more concentrated) solutions we can ignore the effect of the water solvent.
So, pH is 7 from water