I really don't get pH curves. I have to sketch a pH curve for the titration of 40.00 mL of 0.100 M hydrazine, H2NNH2(aq) having a Kb of 3.0x10^-6 by 0.100M HCIO4(aq). I know you can't sketch on here. But can you help me find the first point?

I know its a (SA-WB)

The secret to these curves is to know what you have in solution. At the beginning you have pure hyedrqzine (0.1M) so it is worked just like an 0.1M ammonia solution.

......H2NNH2 + HOH ==> H2NNH3^+ + OH^-
begin..0.1.............0..........0
change..-x..............x.........x
equil..0.1-x...........x...........x

Kb = (H2NNH3^+)(OH^-)/(H2NNH2)
Substitute from the ICE chart into Kb expression and solve for OH, then convert to pH. That gives the beginning of the curve.

Next I would determine the pH for the equivalence point. That is determined by the concn of the salt
H2NNH2 + HCl ==> H2NNH3^ + Cl^-
Set up an ICE chart and do that one.
Everything between the beginning and the equivalence point is a buffer. Calculate moles H2NNH2 to start, calculate moles HCL added, subtract moles H2NNH2-moles HCl to find H2NNH2 remaining, plug into the Henderson-Hasselbalch equation and solve for pH.

Everything past the equivalence point is simply excess HCl. So (H^+) = (HCl) = mole HCl in excess/L soln.
Here is a reference.
http://www.google.com/search?q=pH+curves&hl=en&client=firefox-a&hs=S5i&rls=org.mozilla:en-US:official&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=ZpKlToXLF-qssQLk5aDDBQ&ved=0CCoQsAQ&biw=800&bih=406

Sure! To find the first point on the pH curve for this titration, we need to determine the initial pH of the hydrazine, which is a weak base (WB), reacting with a strong acid (SA), which is HClO4.

The first point on the pH curve occurs when there is only hydrazine present before any titration occurs. Since hydrazine is a weak base, we can treat this as a simple base dissociation problem. We'll use the given Kb value of hydrazine to determine the concentration of hydroxide ions (OH-) produced in the solution.

Here's how you can find the concentration of OH-:

1. Write the balanced chemical equation for the dissociation of hydrazine (H2NNH2):
H2NNH2 ⇌ 2NH2- + 2H+

2. Set up an ICE (Initial, Change, Equilibrium) table. Since hydrazine is a weak base, we can assume that its initial concentration (I) is equal to its initial concentration (0.100 M), and the concentration of OH- is initially 0 M.

| H2NNH2 | 2NH2- | 2H+ | OH-
------------------------------------------------
I | 0.100 M | 0 M | 0 M | 0 M
C | -x | +2x | +2x | +x
E | 0.100 - x | 2x | 2x | x

3. Set up the expression for the base dissociation constant (Kb) of hydrazine:
Kb = [NH2-]^2 / [H2NNH2]

Since [NH2-] = 2x and [H2NNH2] = 0.100 - x, the expression becomes:
Kb = (2x)^2 / (0.100 - x)

4. Substitute the given Kb value (3.0x10^-6) into the expression and solve for x:

3.0x10^-6 = (2x)^2 / (0.100 - x)

Rearrange the equation:
(2x)^2 = 3.0x10^-6 * (0.100 - x)

Simplify:
4x^2 = 3.0x10^-7 - 3.0x10^-6x

Set the equation equal to zero:
4x^2 + 3.0x10^-6x - 3.0x10^-7 = 0

Solve this quadratic equation for x using the quadratic formula or a calculator. It will give you two values for x, but since x represents the concentration of OH-, which must be positive, you will only consider the positive value.

5. Now that you have the concentration of OH-, you can find the pOH by taking the negative logarithm (base 10) of the OH- concentration:
pOH = -log[OH-]

6. Finally, calculate the pH using the equation:
pH = 14 - pOH

Now you have the initial pH of the hydrazine solution, which is the first point on the pH curve.