PH of 8.57x10-10M HNO3 solution?

  • Thread starter erjkism
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In summary: When there is an equilibrium between two substances, the concentrations of each substance will not change with time. This is what my professor meant when he said the problem was more complicated than just changing the concentration of nitric acid.
  • #1
erjkism
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Homework Statement


I have to find the pH of a 8.57x10-10M HNO3 solution.

Homework Equations


HNO3 + H20 <--> H3O+ + NO3-
pH= -log[H3O+]

The Attempt at a Solution


my theory was that the hydronium ion concentration would be the hydronium ion concentration in water (1x10-14 M) plus the concentration of the nitric acid (8.57x10-10 M... because it fully dissociates). then take the negative log of it. i came up with a pH of 6.99

but my chemistry professor told me that i did it wrong and it is actually more complicated and could involve equilibrium. what did i do wrong?
 
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  • #2
Are you sure you mean 10^-10 molar? that is a very low proportion (ie ppb) of acid
 
  • #3
yeah that's what i mean. that's why this problem is getting difficult. if you just change the concentration of nitric acid to the H3O+ concentration, you get a pH above 7.. which is wrong
 
  • #4
erjkism said:
yeah that's what i mean. that's why this problem is getting difficult. if you just change the concentration of nitric acid to the H3O+ concentration, you get a pH above 7.. which is wrong

Not so. Hydronium from the Nitric acid in water will give a pH BELOW 7.

You will need to use the dissociation of water along with the presence of the small concentration of the strong acid, nitric acid.
 
  • #5
The water will not disassociate to produce 1x10^14 M H30+, it will disassociate to produce x M H30 and x M OH. You can then use the total molarity of H3O from the disassociation of water PLUS the disassociation of HNO3 multiplied the the molarity of OH to find Kw, from then you can solve the quadratic and find the pH.
 
  • #6
pH = -log[H+] is a convention that only works if the concentration of the acid is 10^-6 or greater. Below 10^-6, the equation will never give you the correct answer. The case is the same with pOH.

For very dilute concentrations (greater than 10^-6), you need to consider equilibrium.
 
Last edited:

1. What does the pH of 8.57x10-10M HNO3 solution indicate?

The pH of a solution is a measure of its acidity or basicity on a scale of 0-14. A pH of 8.57x10-10M HNO3 solution indicates that it is a very weak acid and therefore has a high pH value.

2. How is the pH of a solution determined?

The pH of a solution is determined by measuring the concentration of hydrogen ions (H+) in the solution. This can be done using a pH meter or through a titration process.

3. What factors can affect the pH of a solution?

The pH of a solution can be affected by the concentration of acids or bases present, temperature, and the presence of any buffer solution.

4. How can the pH of a solution be adjusted?

The pH of a solution can be adjusted by adding acids or bases to the solution, depending on whether the desired result is a lower or higher pH value. Buffer solutions can also be used to help maintain a stable pH level.

5. What are the potential dangers of working with a solution with a pH of 8.57x10-10M HNO3?

While a solution with a pH of 8.57x10-10M HNO3 is considered a weak acid, it is still corrosive and can cause irritation to the skin, eyes, and respiratory system. It is important to handle this solution with caution and wear appropriate protective gear.

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