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# Acid and Base help in deciding which acid and base will be pH=7 at the equivalence point

Submitted by JK_Legend on Sun, 04/20/2008 - 17:11

1. Which of the following titrations would have a pH > 7 at the equivalence point?
a. HI with KOH
B. HCIO4 with NH3
C. HCL with Sr(OH) 2
D. HCOOH with NaOH

2. Which would produce a yellow solution at a pH = 4.0
A. methyl red
B. Methyl violet
C. indigo carmine
D. chlorophenol red

3. A buffer can be made from equal moles of
A. HCl and NaCl
B. HCN and KOH
C. HNO3 and NH3
D. CH3COOH and NaCH3COO

4. Which of the following dissolves inw ater to produce a basic solution?
a. O2
B. SO2
C. NO2
D. MgO
^i know that it's suppose to be a metal oxide that makes basic solution, but theres SO2 and Mg2 which are both metal oxide, so how do i know which one is it?

can someone please explain to me how to do these questions, and also i need a brief explanation of the difference between equivalence point and transition point....and also the definition of a buffer...my tutor say it must be a weak conjugate acid and base pair...and I don't know why....sorry for my stupid questions

i appreciate your help...thank you!  :-*

The equivalence point is the point where you have added just enough acid to exactly neutralize the base (or vice versa).  The transition point is where you get the color change?  If you do a titration of NaOH by HCl the equivalence point is where you have added just enough HCl to exactly neutralize the NaOH that was present.  Since the salt formed by a strong acid (HCl) and a strong base (NaOH) is a neutral salt, the pH of the solution at the equivalence point would be 7.  However, methyl orange has a transition point (the pH at which it goes from yellow to orange) below 7 while phenolphthalein has a transition point (the pH at which it goes from colorless to pink at about 9).  Since the change in pH near the equivalence point for the titration of a strong acid and a strong base is very large, either indicator can be used and their transition points give a good approximation of the equivalence point for the titration.

A buffer is a mixture of a weak acid and its conjugate base (say acetic acid/acetate ion) that is good at maintaining a fairly constant pH even when acid or base is added to it.  This is because the acid (in this case acetic acid) can neutralize any additional OH- ions that are added to the solution.  However, if extra acid is added (H+ ions) the acetate ion removes them from solution by reacting with them to form acetic acid.

When I was first learning this I was confused by this, how could absorbing H+ to form acetic acid make the solution less acidic?  The answer is that what causes a solution to be acidic is not how much acetic acid is present, but how many H+ ions are present.  So when the H+ ions are removed by the acetate ion, the acidity is reduced even though acetic acid is formed.

Well, here are some hints for your questions:
1. Which of the following titrations would have a pH > 7 at the equivalence point?
To get a pH greater than 7 you would need to form a basic salt.  Basic salts are formed when a weak acid is neutralized by a strong base.  So which of your choices are a weak acid and a strong base.

2. Which would produce a yellow solution at a pH = 4.0
You would need to consult a chart which lists the pH changes.  If you were assigned the question you must have a chart in your textbook that would list these indicators.  Or you could try googling acid-base indicators.

3. A buffer can be made from equal moles of
a weak acid and its conjugate base
or
a weak base and its conjugate acid

4. Which of the following dissolves in water to produce a basic solution?
You are correct that metal oxides are basic anhydrides.  However, you are incorrect in saying that you have two metal oxides.