[naturelover]

Hi to whoever is reading this and thank you!.

I'm having a bit of trouble understanding some "Basic" chemistry- sorry I'm an environmental science major and the last time I touched chemistry was in GCSE. I was wondering if anyone could please kindly and easily explain to me the follows? (I've asked my Professor, but he made it more confusing.. and it felt like he expected me to know this.. I sort of feel dumb for not understanding.. sorry). I also apologize if my questions are long.

1) (Ionization of acids and bases): What are the concentrations of proton and hydroxyl. ion in a tap water sample of pH 6? What is the pH after the tap water sample (100mL) is mixed with a strong acid solution (100mL, 20mM HCL)?

--> I know this: pH= -log (H+)=6, (H+)=10^-6, (OH-)=10-8
--> HCL=H+ +Cl-

now what I don't understand is how it became : moles of H+ are 20x 0.001 (where did this come from?) x0.1 (why must /1000 to change this into Litres? )=0.002moles
vol: 100+100=200ml = (/1000=0.2L)

2) My next question is can you please explain to me very simply what is initial values, equilibrium values and how do I find/calculate them, oh and also with theromodynamic equilibrum relations (activity and temp). I know I will need this to calcuate equilibrium state... I don't know what x, w, z, y are.. I'm assuming that they are the species? and How do I calculate the approximate solution? . Eg. "If 1.7g NH3 is added to distilled water to make 1L of solution, what will be the ammonium conc? pH?"

I know that we will not have a computer to work with, so this will have to be done via assumption. Where does the inital value come from or how do I find it? I think the chemical equation to this formula is : NH3+H20 = NH+4 + OH-.

Then the equation is KB= (NH+4 + OH-/NH3) (now how does it become this:) = x(10-7+x)/0.1-x =1.75x10^-5.

Where does the 1.75x10^-5 come from? the question never had any of this? also how do you assume a pH or calculate it if I'm just assuming?

3) Calculating Equilibrium constant: I know that the free energy of this reaction. but if I had a question "Determine the first ionization constant for carbonic acid at 25c from free-energy considerations. "

--> The first step would be to have the eqattion of free energies for the first ionization (?): so.. H2C03=H+ +HCO3

Then from the table of "standard enthalpies and free energies  f formation at 25c.. i will find the no for the solution?

deltaG0298: -623.4kJ = 0 (now why isn't H+ a 1? why 0?)+ -587.1kJ= 36.3kJ

In K -deltaG0/RT= -36300/8.314(298) = -14.65 correct? Even if I log it on a calculator the answer is only -1.12..

But how does the answer given from my professor is K=4.34x10-7 at 25c?


4) How do I calculate a chemical tritation? I understand that its an old method of characterizing ions. But I don't undersatnd the equations and the steps given.. I find this very complicated, epecailly when you have the Vo/Co etc. I'm stuck on this question that I can't seem to understand or calculate let along finding the answer?

" Consider the titration of 500mL of 0.01 M boric acid with 0.5M NaOH. Calculate the pH at different points in the titration to show the relative changes in pH with respect to the vol. of titrating solution added (Ignore the activity correction. KA1= 5.8x10^-10 at 25c)".

Can you please if possible explin the steps I need to do? In class there was something about finding the midpoint, end point titration, weak base with strong acid and weak acid with strong base. This to me is all confusing, let alone the equations. I went online to check how to do it, and any easier way to understand.. but.. I couldnt find any. Let alone how to calcuate the buffer and buffering capacity....

5) My final question, is oxidation-reduction reactions (equilibrium relationships).

How do you calculate a half reaction for the reduction of NO3- to NH4+ (based on one electron).

I understand you look at the tables to find the  numbers..  so it would be:

1/8 NO3- + 5/4 H+ +e = 1/8 NH4+ +3/8 H20 (how do you balance this equation?)

and how come the equation would require to use free energy at standard state or the standard electrod potential for the reaction? something like PE or PH.

Thank you very much, I apologizing for asking so many questions, but I"m having a hard time understanding.. if you also know any websites that could help me understand, please also let me know.

Thank you for you kind *delete me*

[Borek]

Sorry, no time for more elaborate explanation, when it comes to titration please see:

http://www.titrations.info/titration-calculation

(chances are it will also help you with n=CV calculations in general).

1.75x10^-5 is just a value of Kb for ammonia, and is taken from tables (determined experimentally).

My bet is that 20×0.001 is just conversion from mM to M. 0.1 is 100 mL expressed in L. To convert from mX to X you divide by 1000.