# what is the PH of 1 x 10 ^ -8 M of HCl

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#### dentalstudent2021

hey guys, there is a trick to it that I can't seem to grasp.

the answer is 6.98 btw rather than 8.

This is an important trick that must be remembered for the real DAT.

When the concentration of HCL is smaller than 1x10-7 you have to consider water!

Water has a [H] of 1x10-7 and thus if you add [HCL] 1x10^-8 you actually have a combined [H] of 1.1x10^-7. the the negative log of this is just under 7 ===> 6.98

• tooth_hurty, 510586, virtualmaster999 and 5 others
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#### dentalstudent2021

thanks and congrats w4nt2baDDS on the DAT.

so would this work for a base ?

Thanks, jjj7236!

Yup, it would work for bases too. Just make sure you're careful in terms of checking whether the are asking for pOH or pH.

Also, remember HCL is a strong acid and thus completely dissociates. With weak acids and weak bases the dissociated concentrations are usually much less than the initial acid and base concentration. Thus to find the [H] concentration of 1x10-8 [acetic acid (CH3OOH)] you would have to use to Ka value to determine the amount of [H] that dissociated. You cannot just add 1x10^-8 to 1x10^-7 like I did before... because the [H] would be even smaller than 1x10^-8... but just have in mind that this would be less additional [H] and thus the pH would be even closer/VERY close to 7

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#### virtualmaster999

Thanks, jjj7236!

Yup, it would work for bases too. Just make sure you're careful in terms of checking whether the are asking for pOH or pH.

Also, remember HCL is a strong acid and thus completely dissociates. With weak acids and weak bases the dissociated concentrations are usually much less than the initial acid and base concentration. Thus to find the [H] concentration of 1x10-8 [acetic acid (CH3OOH)] you would have to use to Ka value to determine the amount of [H] that dissociated. You cannot just add 1x10^-8 to 1x10^-7 like I did before... because the [H] would be even smaller than 1x10^-8... but just have in mind that this would be less additional [H] and thus the pH would be even closer/VERY close to 7
So with a strong base, you would still add 1x10^-7 because the H+/OH conc would be the same for water?

And for weak acid, would you do H+ = sqrt( HA*Ka) and add that to 1*10^-7?

So with a strong base, you would still add 1x10^-7 because the H+/OH conc would be the same for water?
Think you have the right idea, but might have just miss typed:

With a strong base, you would sum the 1x10^-7 concentration of [OH] within the water with the [OH] concentration produced from the base which as far as the DAT is concerned is equal to the concentration of the [Strong Base].

And for weak acid, would you do H+ = sqrt( HA*Ka) and add that to 1*10^-7?
Yup!

• virtualmaster999

#### virtualmaster999

Think you have the right idea, but might have just miss typed:

With a strong base, you would sum the 1x10^-7 concentration of [OH] within the water with the [OH] concentration produced from the base which as far as the DAT is concerned is equal to the concentration of the [Strong Base].

Yup!
Awesome thanks! Glad I found this out..wouldve missed it.

• #### virtualmaster999

Just to double check: you mean this works for bases as well when the BASE concentration is also less than 1*10^-7?

Just to double check: you mean this works for bases as well when the BASE concentration is also less than 1*10^-7?
Yes.

In all honesty though, you can always sum the the values together... which ever is larger (water or the base ---- water or the acid) is what will dominate the pH/pOH outcome.

• virtualmaster999
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#### dentalstudent2021

Thanks, jjj7236!

Yup, it would work for bases too. Just make sure you're careful in terms of checking whether the are asking for pOH or pH.

Also, remember HCL is a strong acid and thus completely dissociates. With weak acids and weak bases the dissociated concentrations are usually much less than the initial acid and base concentration. Thus to find the [H] concentration of 1x10-8 [acetic acid (CH3OOH)] you would have to use to Ka value to determine the amount of [H] that dissociated. You cannot just add 1x10^-8 to 1x10^-7 like I did before... because the [H] would be even smaller than 1x10^-8... but just have in mind that this would be less additional [H] and thus the pH would be even closer/VERY close to 7
that would be the ka = (H)(A-) / (HA) equation right ?

can you give a instance where they would give an acid and ask for POH ? if thats what you meant. or if they give base and ask for pH.

that would be the ka = (H)(A-) / (HA) equation right ?

can you give a instance where they would give an acid and ask for POH ? if thats what you meant. or if they give base and ask for pH.
Yup, that is the Ka value I was talking about.{Equilibrium (K) expression for an acid (a)}

Sample Question...(if you want real exposure you need to work Destroyer and Chad's questions)

What is the pOH of an aqueous solution of 1x10^-8 HCL..

• jjj7236

#### virtualmaster999

Yup, that is the Ka value I was talking about.{Equilibrium (K) expression for an acid (a)}

Sample Question...(if you want real exposure you need to work Destroyer and Chad's questions)

What is the pOH of an aqueous solution of 1x10^-8 HCL..

#### orgoman22

##### DAT DESTROYER
Vendor
15+ Year Member
hey guys, there is a trick to it that I can't seem to grasp.

the answer is 6.98 btw rather than 8.
The concentration of acid here is very low and therefore you need to consider the contribution of water.

Let's pretend you forgot to do this and you took the negative log of 1x10 exp -12. You would got an answer of 12, correct ? This can not be so, since an HCl solution is acidic and the pH must be under 7!!!. This should throw up a red flag signifying you have encountered a trick problem, in such a case the pH will be extremely close to 7, this trick will work perfectly for the DAT.

ALWAYS be careful of answers that make no sense.

Hope this helps

Dr. Romano

• jjj7236, DentalGirl5, alexis2010 and 1 other person

#### virtualmaster999

The concentration of acid here is very low and therefore you need to consider the contribution of water.

Let's pretend you forgot to do this and you took the negative log of 1x10 exp -12. You would got an answer of 12, correct ? This can not be so, since an HCl solution is acidic and the pH must be under 7!!!. This should throw up a red flag signifying you have encountered a trick problem, in such a case the pH will be extremely close to 7, this trick will work perfectly for the DAT.

ALWAYS be careful of answers that make no sense.

Hope this helps

Dr. Romano
This applies when the OH concentration is less than 1x10^-7 right? Say OH= 1*10^-8. pOH=8, and you would think this seems odd for a base.

#### orgoman22

##### DAT DESTROYER
Vendor
15+ Year Member
This applies when the OH concentration is less than 1x10^-7 right? Say OH= 1*10^-8. pOH=8, and you would think this seems odd for a base.
A basic solution would be done in a similar way.......If the base concentration was say...1 x10 exp -10........... You may have erroneously thought the pOH was 10...thus the pH would be 4

Clearly this is not possible......Again the answer would be approx 7 !!!!!

Keep hammering away !

• virtualmaster999 and DentalGirl5
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#### dentalstudent2021

A basic solution would be done in a similar way.......If the base concentration was say...1 x10 exp -10........... You may have erroneously thought the pOH was 10...thus the pH would be 4

Clearly this is not possible......Again the answer would be approx 7 !!!!!

Keep hammering away !
Thanks Dr. Romano. so for this one, what would you estimate the pOH then if pH is around 7.

#### orgoman22

##### DAT DESTROYER
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