Homework Help: ap chemistry

Posted by chris on Thursday, October 2, 2008 at 10:19pm.

please explain titration problems. I'm a total noob at this and am trying to answer some prelab questions. Examples:

1. How many mL of a 0.800 M NaOH solution is needed to just neutralize 40 mL of a 0.600 M HCl solution?

2. You wish to determine the molarity of a solution of NaOH. To do this, you titrate a 25.00 mL aliquot of your sample, which has had 3 drops of phenolphthalein indicator so it is pink, with 0.1067 M HCl. The sample turns clear (indication the NaOH has been neutralized with the HCl solution) after the addition of 42.95 mL of the HCl. Calculate the molarity of the unknown concentration of NaOH solution.

ap chemistry - GK, Thursday, October 2, 2008 at 10:52pm
Both questions can be answered by substituting the informatin given into:
M_aV_a = M_bV_b
M_a = molarity of the acid (HCl)
V_a = volume of acid
M_b = molarity of base (NaOH)
V_b = volume of base
The unknown in #1 is V_b
The unknown in #1 is M_b
ap chemistry - DrBob222, Thursday, October 2, 2008 at 11:27pm
I want to make a couple of points here.
1. The equation provided by GK works quite well for both of these problems; however, it is good ONLY for those equations in which the mol ratios of acid to base is 1 as they are in HCl and NaOH. If H2SO4 was being titrated with NaOH it would not work (at least without modification).
2. The second point is reserved for the second problem itself, not in the way it was worked. Specifically, I don't like the wording of the problem. I want to point out that the solution was pink initially and it was titrated until the solution was COLORLESS. Colorless is the absence of color; clear is the absence of turbidity. The solution changed COLOR from pink to colorless.
Have a good day.

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