To make a long story short, my youngest daughter decided to endure 
through 4 semesters of chemistry, and now is paying up for it. She is 
a talented little girl, but science is not her thing and it hurts for 
me to see her study so hard but understand so little. It runs in the 
family. I wanted to help myself, but me and my wife are no scientists 
at all, plus it was so long ago. Tutoring is very very weak at where 
she goes, and we know few friends around here who are capable of 
helping, most of which already are looking at us the wrong way for 
asking so much. I decided to open my pocket and let you, Chemistry 
folks, help us out. I secretly took the next set of questions from my 
daughters assignment section, and here they are. I understand that 
many (or most) of you would have the problem to giving direct help 
like this, "learning and understanding the material is the most 
important", right ? Wrong. Only but a few of you would understand the 
pain I feel when I see my daughter quietly cry in her room over her 
low assignment grades after hours and hours of studying daily. 
  
Below I copied down some of the questions and choice answers that came 
along. I tried my best to copy down everything number to number, word 
for word. Its a little bit difficult to transfer everything to text, 
but I tried. Please feel free to clarify if you did not understand a 
part. 
  
I am asking for a list of answers to the problems, in numerical + 
content format, as such : 
  
Question 1: Answer 3. The number of moles is 4.0.  
Question 2: Answer 5. The reaction is reversed.  
  
Time is an issue, but I would like the answers to be correct. I do not 
want to check my daughters answers and tell her that one of her 
correct answers is incorrect simply because what was given to me is a 
wrong answer. Once again I apologize for such rude question, but I am 
desperate, and I ran out of all other options. I am also considering a 
big tip for those who'll work hard on these, do all (or most) of the 
questions, provide the correct answers and hopefully as soon as 
possible. 
  
Thank you once more. You are my last hope. 

1) Which of the following is the best chemical
definition of pure (or neutral) water?

1. water that has a pH equal to ZERO
2. water that has been freshly pumped from well
3. water that has a pH exactly equal to 7.00
4. water that has a pOH slightly above 7.00 equal to the concentration of OH-
5. water that has a pH equal to Kw
6. water where the concentration of H+ is

2) What is the pH of a 8.80 * 10^-5 M HNO3
solution?

1. 3.06
2. 4.06
3. 4.79
4. 5.06
5. 5.63

3) The buildup of lactic acid in muscles causes
pain during extreme physical exertion. The
Ka for lactic acid (C2H5OCOOH) is 8.4 *
10^-4.
Calculate the pH of a 0:06 M solution of
lactic acid.

1. 1.926
2. 2.043
3. 2.149
4. 2.174
5. 2.297
6. 3.533
7. 6.348
8. 11.851

4) The ratio of NH3 to NH4+ in an aqueous
solution is 4.0. The NH3 concentration is
3.2 * 10^-3 M and Kb(NH3) = 1.8 * 10^-5.
What is the solution pH?

1. 4.14
2. 9.86
3. 5.35
4. 8.65
5. Need to know solution volume to determine the pH

5) Calculate the pH of 0.10 M H3PO4 solution.

1. 1.77
2. 1.62
3. 1.57
4. 1.67
5. 1.72

Hi jwheel!
Here are the answers to your questions.

1) Which of the following is the best chemical definition of pure (or
neutral) water?

The answer is 3. water that has a pH exactly equal to 7.00 

You can check this definition at the following page:

Acids and bases
http://www.visionlearning.com/library/module_viewer.php?mid=58


2) What is the pH of a 8.80 * 10^-5 M HNO3 solution?

The answer is 4.06 (answer 2). Since HNO3 is a strong acid that fully
dissociates, we get 8.8*10^-5 M H+ from the solution. So now we can
simply apply the formula for the pH:

pH=-log(H+)
  =-log(8.8*10^-5)
  =4.06

More information on pH at:

Defining pH and pOH
http://www.ausetute.com.au/phscale.html


3) The buildup of lactic acid in muscles causes pain during extreme
physical exertion. The Ka for lactic acid (C2H5OCOOH) is 8.4*10^-4.
Calculate the pH of a 0:06 M solution of lactic acid.

The correct answer is 2.149 (answer 3). The formula to calculate the
pH given the concentration and dissociation constant (Ka) is given in
the following link.

Weak Acid K
http://www.mpcfaculty.net/mark_bishop/weak_acid_equilibrium.htm


4) The ratio of NH3 to NH4+ in an aqueous solution is 4.0. The NH3
concentration is 3.2 * 10^-3 M and Kb(NH3) = 1.8 * 10^-5. What is the
solution pH?

The correct answer is 9.86 (answer 2).

In this case, we have:

Kb = [OH-][NH4+]  =1.8*10^-5
     -----------
       [NH3]

Since the ratio of NH3 to NH4 is 4.0, we get:

Kb = [OH-][NH4+]  =1.8*10^-5
     -----------
          4

Therefore, the concentration of OH- is 1.8*10^-5*4 = 7.2*10^-5 M. So
the pOH is -log(7.2*10^-5)=4.14. Finally, we use the fact that
pH=14-pOH to get pH=9.86.


5) Calculate the pH of 0.10 M H3PO4 solution.

The correct answer is 1.57 (answer 3). A step by step explanation of
how to arrive to this number can be found in the last slides of the
following Powerpoint presentation. Once you obtain the values of x, y
and z for this particular case (this is the notation used in the
presentation), just sum these values. The sum is the H+ concentration.
Now, in order to calculate the pH, we use the standard formula -log10
of this sum. This will give 1.57.

http://www.chem.neu.edu/courses/reiff/download/Eb_Chpt16.ppt


Google search strategy
calculate ph
phosphoric acid ph
acid dissociation constant
base dissociation constant


I hope this helps! If you have any doubts regarding my answer, please
request a clarification before rating it. Otherwise I await your
rating and final comments.

Best wishes!
elmarto

Thank you very much. Would you be able to answer any of the other sets ?

Thanks for the rating comments and tip! I hope to see you around
Google Answers real soon.

Best wishes and good luck to your daughter!
elmarto