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Need help with a pH question watch

1. hi I would like some help with part d please. Attachment 590076590080 here is my working . I don't get how you can just use the concentration of the acid as the conc of the h+ ions because they are different right ? without a Ka value, how are to supposed to find conc of h+ since ph is -log of h+. also I know the answer is -, how can ph be - tho??

Thanks
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2. here is a better picture
3. (Original post by coconut64)
here is a better picture
Yes, I can feel your pain ...

It is perfectly feasible for pH to be negative. Any concentration of strong acid greater than 1 will have a negative pH.

In fairness, the pH scale was never designed to work with concentrated acids as one of the basic assumptions - that strong acids are 100% dissociated in aqueous solution does not hold for very strong solutions.

I imagine that they are expecting you to assume 100% dissociation and then blindly apply the pH = -log[H+].

Taking the question at face value, 65% of 1400 = 910g
Moles of nitric acid = 910/63 = 14.44 mol
Hence solution is 14.44 molar in H+ ions = pH -1.16

If we apply equilibrium principles, there is simply not enough water for the full dissociation, assuming:

HNO3 + H2O <==> NO3- + H3O+
4. (Original post by charco)
Yes, I can feel your pain ...

It is perfectly feasible for pH to be negative. Any concentration of strong acid greater than 1 will have a negative pH.

In fairness, the pH scale was never designed to work with concentrated acids as one of the basic assumptions - that strong acids are 100% dissociated in aqueous solution does not hold for very strong solutions.

I imagine that they are expecting you to assume 100% dissociation and then blindly apply the pH = -log[H+].

Taking the question at face value, 65% of 1400 = 910g
Moles of nitric acid = 910/63 = 14.44 mol
Hence solution is 14.44 molar in H+ ions = pH -1.16

If we apply equilibrium principles, there is simply not enough water for the full dissociation, assuming:

HNO3 + H2O <==> NO3- + H3O+
Thanks. When you say 100% dissociation , does that mean h+=hno3 for the concentration ?
5. (Original post by coconut64)
Thanks. When you say 100% dissociation , does that mean h+=hno3 for the concentration ?
yes
6. (Original post by coconut64)
Thanks. When you say 100% dissociation , does that mean h+=hno3 for the concentration ?
You might also like to consider what the pH of a very dilute solution of acid might be. For example, nitric acid with a concentration of 10^-9 mol/dm3
7. (Original post by TeachChemistry)
You might also like to consider what the pH of a very dilute solution of acid might be. For example, nitric acid with a concentration of 10^-9 mol/dm3
Nearly 1?
8. (Original post by coconut64)
Nearly 1?
Remember that [H+][OH-] = 1 x 10-14
9. (Original post by charco)
Remember that [H+][OH-] = 1 x 10-14
Isn't that kw ? I thought in this case, you will do -log h+ to find pH ... Thanks
10. (Original post by coconut64)
Isn't that kw ? I thought in this case, you will do -log h+ to find pH ... Thanks
Yes, and kw is a CONSTANT
11. (Original post by charco)
Yes, and kw is a CONSTANT
Why can't you use pH=-log h + though ? Thanks
12. (Original post by coconut64)
Nearly 1?
Concentration of 10-9 so VERY dilute
13. (Original post by coconut64)
Why can't you use pH=-log h + though ? Thanks
The learning point here is that the hydrogen ion concentration due to the dissociation of water is 10-7 which is far greater than the hydrogen ion concentration from the nitric acid.

Plugging in 10-9 into the pH = -log [H+] equation would give a pH of 9 which is obviously impossible. An acid must have a pH of below 7. We have to consider the water.

The hydrogen ion concentration is 0.000000101

This gives a pH of 6.996.

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