CHoosing an indicator and explaination?

[h20h]

Q)  Consider the indicators below and select one that would be best to indicate the endpoint of a titration of HN02 with NaOH. Explain your choice.

Methyl Orange  Red:  Below pH 3.1   to Yellow above pH 4.4

Bromocresol Purple:   Yellow below pH 5.2 to Purple above pH 6.8

Phenolphthalien:   clear below pH 8.3 to Pink between pH 8.3-10

I am stuck in trying to find a way to explain it in terms of using the HN02 and the NaOH.  I know that HN02 is a weak acid and that NaOH is a strong base...I would think I could write that the titration of the weak acid would leave excess OH ions in the solution, pushing the titrated acid solution to the basic side?  But then I was told to talk about the conjugate base of HN02 which is NaNO2 and that is slightly acidic?

the reaction would be :  HN02 + NaOH = NaNO2 + H20

is that right?  the above explaination and the equation...any tips on what to write, say. etc???

Thanks
H20h

[quantumboredom]

You should look at a titration curve for a weak acid by a strong base.


When all of the moles of HNO2 are converted to the conjugate base NO2- you have reached your endpoint.

At that point , any excess NaOH will produce OH- in the solution.  But just before this point all the HNO2 is now NO2- so you can figure out the pH at this point by treating  NO2- as a base and solve for pOH.... subtract from 14 and get the pH

Do you have your ka or kb for NO2-

but to be more concise, all the HNO2 is converted to NO2- at the endpoint. Solve for the pH considering NO2- as a weak base. Whichever indicator contains this pH in its range, is the indicator you should choose.

[h20h]

We are not given any information other then just the question that I stated...In a weak acid strong base titration curve the resulting equivalence point should be higher then 7?  I am just wondering if that is the case in this situation....this would be due to the excess OH- that is present being that the strong base will dissociate completely?  So I should choose phenolphthalein?  I am confused by what you wrote about finding the pH since I was not given any information other than just the question?

Thanks and I appreciate your help and let me know what you think?

[quantumboredom]

Ah I see... there is a range so make up a initial molarity

say .1 molar  as long as the molarity is not too low the pH will be close to one of the ranges above

so answer this... you have .1 M NO2-  , look up the Kb and solve for pH

what range will you get.....

Also since at the endpoint the only species you have left is a base , then the pH of the solution at that point will be above 7.

And do me a favor. tell you friends to come by and join. Read the site suggestions for ways to help out.

thanks

[h20h]

Ok, so I am correct in understanding that the range will be above 7?  So the only choice would be phenolphthalein....correct?'

[Chemistry Tutor]

Yes, that is correct. The chemical species determining the pH at the endpoint will be NO2-, which is a base. Therefore the pH will be above 7.

If you don't mind please post in the Meet & Greet so we can see where you are from.