# Chemistry 12

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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)

• Chemistry 12 -

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.

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