Finding amount of NaN02 needed to raise the a solution's pH to 5

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In summary, to achieve a pH of 5.00 in a .200 M solution of HNO2 with a Ka of 4.0^-4, around 8.5 M of Sodium Nitrite must be added to shift the equilibrium to the left and decrease the concentration of (H+) ions. The equilibrium concentration of HNO2 can be assumed to be around .200 M due to the high excess of NO2- ions.
  • #1
rokku
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What amount of sodium Nitrate must be added to 500 ml of .200 M solution of HN02. Ka=4.0^-4


Ka=(H+)(A-)
--------
(HA)



Ph of 5 = 1.0^-5 (H+) so, (1.0^-5)(x)
---------- = 4.0^-4 , however X turns out to be way too big
.200
 
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  • #2
Sorry, it is barely unreadable, try to use LaTeX to format the equation.

If I understand correctly what you did, your approach looks OK. Nothing strange in high amount of nitrate required - you want to get to pH which is 1.6 unit from the pKa, that requires around 101.6 more nitrate than there is nitrous acid.
 
  • #3
Sorry about that, let me try again. Besides the obvious spacing issues I incorrectly mention Sodium Nitrate when it really should be Sodium Nitrite.

At first the pH of the undisturbed weak acid HN02 is around .69. I found it using a simple ICE table and the initial concentration HN02 (.200 Molar) plus the Ka (4.0^-4). When Sodium Nitrite is added, the salt completely dissociates and you end up with a common ion (NO2-) which would push the equilibrium to the left and result in less (H+) ions and a higher pH. Because we know what the desired pH is (5.00) we know that at equilibrium there should be the Antilog of 5, or 1.0^-5 M of H+. We also know that there has to be a new concentration of NO2- ions which is designated as X. Here is where I get stuck, what would the equilibrium concentration of HN02 be?

The formula to use would be Ka= (H+)(A-)/(HA)----> 4.0^-4=(1.0^-5)(X)/(equilibrium concentration of HN02?)
 
  • #4
You can reasonably safely assume equilibrium concentration of HNO2 to be 0.2. High excess of NO2- shifts the dissociation far to the left.

My approximate calculations show that you need around 8.5M of NaNO2 (that ignoring ionic strength of the solution, which is pretty high).
 
  • #5
Borek said:
You can reasonably safely assume equilibrium concentration of HNO2 to be 0.2. High excess of NO2- shifts the dissociation far to the left.

If High excess of NO2- shifts the dissociation far to the left, wouldn't the equilibrium concentration of HNO2 be significantly larger than its initial of .200?
 
  • #6
No, you don't have a source of H+ (other than water, but it is an acid many orders of magnitude weaker than HNO2 itself).
 
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  • #7
Ah I see, thank you.
 

FAQ: Finding amount of NaN02 needed to raise the a solution's pH to 5

How do I calculate the amount of NaN02 needed to raise a solution's pH to 5?

The amount of NaN02 needed to raise a solution's pH to 5 can be calculated using the Henderson-Hasselbalch equation. This equation takes into account the initial pH of the solution, the desired pH of 5, and the pKa of NaN02. By rearranging the equation and solving for the concentration of NaN02, the amount needed can be determined.

What is the pKa of NaN02?

The pKa of NaN02 is approximately 3.8. This value may vary slightly depending on the source and conditions, but it is typically in this range.

Can I use any other base besides NaN02 to raise the pH to 5?

Yes, there are many bases that can be used to raise the pH of a solution to 5. However, the amount needed will vary depending on the pKa of the base and the initial pH of the solution. It is important to calculate the amount needed for the specific base being used.

How accurate is the Henderson-Hasselbalch equation for calculating the amount of NaN02 needed?

The Henderson-Hasselbalch equation is a reliable method for calculating the amount of NaN02 needed to raise the pH of a solution. However, it is important to note that this equation assumes ideal conditions and may not account for other factors that could affect the final pH, such as temperature or impurities in the solution.

Is there a specific method for adding the NaN02 to the solution to raise the pH to 5?

The method for adding NaN02 to the solution will depend on the specific experiment or application. In general, it is recommended to first dissolve the desired amount of NaN02 in a small amount of water and then slowly add it to the solution while monitoring the pH. This will help prevent overshooting the desired pH and allow for adjustments if needed.

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