Acids and Bases P1 Help!

[Scienuk]

pKw for water at 10^oC = 14.54. What is the pH of pure water at this temperature?

a) 6.73

b) 7.00

c) 7.27

d) 7.54

The answer is c, I fully understand how to get to this answer, however I don't know how to do it without a calculator due to the difficulty of the arithmetic involved.

Which is correct for a weak acid, HA, with concentration 0.01 mol dm^-3 at 298K?

a) [H^+] < 1x10^-2

b) pH < 2

c) [OH^-] < 1x10^-12

d) pOH > 12

The answer is A, why is that? isn't the concentration of H^+ exactly 0.01 mol dm^-3 as stated in the question? why is it less.

Another question I have is whether the mixing of different acid/base indicators is included in the syllabus, if so, may someone please explain it?

Thanks!

 

Edited April 12, 2017 by Scienuk

[kw0573]

I am not sure whether you are SL or HL, and these are not exactly easy questions per se, but are very important and essential conceptual problems. You should read the book more and try to solve more questions like these if you don't get them. I want to say that acid/base indicators are a concept, it is merely an application of weak acids/bases. For example, you cannot ask "is mixing of HF and NaHSO₄ in the syllabus", because it is just an application. Please kindly include what questions you have seen acid/base indicators so we can help you identify the concepts involved. 

1) 14.54 / 2 = 7.27, or when pKa of water = pKb of water. pKa + pKb = pKw. 

2) Weak acid means the dissociation reaction HA -> H⁺ and A^- does not reach completion. That means the [H⁺]_final is less than [HA]_initial

 

 

[Scienuk]

5 hours ago, kw0573 said:

I am not sure whether you are SL or HL, and these are not exactly easy questions per se, but are very important and essential conceptual problems. You should read the book more and try to solve more questions like these if you don't get them. I want to say that acid/base indicators are a concept, it is merely an application of weak acids/bases. For example, you cannot ask "is mixing of HF and NaHSO₄ in the syllabus", because it is just an application. Please kindly include what questions you have seen acid/base indicators so we can help you identify the concepts involved. 

1) 14.54 / 2 = 7.27, or when pKa of water = pKb of water. pKa + pKb = pKw. 

2) Weak acid means the dissociation reaction HA -> H⁺ and A^- does not reach completion. That means the [H⁺]_final is less than [HA]_initial

 

 

Thank you,

But I didn't quite understand your answer for the first question, what I did to obtain it was convert pKw to Kw, square the value of Kw and then calculator the negative log of the answer to get to 7.27, but it seems impossible to do it without a calculator. If you can please explain why you divided the pKw by two, that'll be great.

[kw0573]

10 hours ago, Scienuk said:

Thank you,

But I didn't quite understand your answer for the first question, what I did to obtain it was convert pKw to Kw, square the value of Kw and then calculator the negative log of the answer to get to 7.27, but it seems impossible to do it without a calculator. If you can please explain why you divided the pKw by two, that'll be great.

First I can show the calculation is equivalent,
You first raised 10 to the power of -x, took square root, then took negative log, which is -log₁₀((10^-x)^0.5) = log₁₀(10^-0.5x) = 0.5 x, which is equivalent to dividing by two.
Now why I divided by two is same logic as what you did. If K_a K_b = K_w then pK_a + pK_b = pK_w, you used K_a = K_b, I used pK_a = pK_b