Achieve Week 2 #7

[Jiayi_Cao_3E]

Hello!

This question has been asked multiple times but it seems like everyone has different values for the initial concentration. I have 0.016M for the initial concentration of NaClO. I think I followed the steps correctly but I cannot get the correct answer for some reason. This is the question:

HClO is a weak acid ( Ka=4.0×10−8 ) and so the salt NaClO acts as a weak base. What is the pH of a solution that is 0.016 M in NaClO at 25 °C?

And these are my steps:
Calculate Kb = Kw/Ka = 2.5 x 10^-7
Setting up the ICE table and getting the expression x^2/(0.016 - x) = 2.5 x 10^-7
I calculated x = 6.3 x 10^-4. I ignored the -x for 0.016M since it is more than 1000 times larger than the Kb, but I also used the quadratic equation and obtained the same x to at least the hundredth decimal.
I then used -log[6.3 x 10^-4] and got 3.2 for pOH and I subtracted it from 14 and got 10.8 for pH. This was marked incorrect and I don't really know why.

Anything helps!

[Brian Diehl 2B]

Hi Jiayi,

Your setup is 100% correct, but you may have misread the scientific notation for the calculated value of x as I got it as 6.3x10^-5 instead of x10^-4. I reached a final pH value of 9.80, so the error just offset your answer by a factor of 10.

[Clarissa Le 1I]

As Brian said, I think you might have made a mistake in finding X. Using the values you were given, I believe X should've been 6.3 x 10^-5, but all your steps otherwise seem fine!

[Jiayi_Cao_3E]

Thanks! Hope I don't make mistakes like that anymore.