[Janderson]

Hi,
I need some minor help with three chemistry quesions.

1) The molarity of an aqueous solution of NaOH is 0.0024 M.
(a) What is the concentration of H3O+ in this solution?
(b) What is the pH of the solution?
(c) What is the OH- concentration in the solution?

For each answer, the tolerance is +/- 1 in the 2nd significant digit, and this makes me nervous because I already got a similar question wrong because of the significant digits. So, now I'm hoping you'll be able to correct my answer, because I'm too scared to submit it:

a) [H3O+] = 3.98e-12 M
b) pH = 11.4
c) [OH-] = 2.4e-3 M
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2) What are the concentrations of all the solute species in a 0.81 M solution of hydrofluoric acid, HF?
(a) [H3O+], M;
(b) [OH-], M;
(c) [HF], M;
(d) What is the pH of the solution? For HF, Ka = 6.8 x 10-4.

I don't know what it's asking when it asks for the concentrations of all the solute species, because somehow the reaction that I made looks like: HF + H2O <==> F- + H3O+ and the question asks for [OH-] so I'm confused. What are the steps for working out this question?
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3) Consider a galvanic cell that has the following half-equations:

MnO2 + 4H+ + 2e- Mn2+ + 2H2O
EΘMnO2 = +1.23 V

PbCl2 + 2e- Pb + 2Cl-
EΘPbCl2= -0.27 V

I need to work out the standard cell potential for this reaction, but I don't know what the full reaction is? These half reactions have been set up in a way that makes them both look like reduction reactions, and I don't know which one needs to be reversed?
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Thank you,
Jess.

[Hunter2]

Think of the ionic product of water. To ask for H⁺ in an alkaline solution and vise versa is effected by this. pH 7 = 10^-7 H⁺ but also 10^-7 OH^-