# Finding PH

#### studiot

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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• Helly and topsquark

#### Helly

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 ?

#### studiot

In (1) There are 10−7mol of HCl ?
No

I study HH equation. So, i am supposed to be able to find pH of them.
You do not need the HH equation.

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#### Helly

No

You do not need the HH equation.
How do you count for mol of HCl on (1) ?

#### studiot

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 ?

#### Helly

Check question (1) carefully.

What is 0.00000001 ?
10^-7 is moles of HCl
[HCl]=10^-8

#### studiot

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.

• Helly

#### Helly

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

#### studiot

• topsquark