Determine the ph when 2.00ml of additional Ca(OH)2 is added after the equivalence point.
Started with 20.00ml of HNO2 and it is titrated with 1.50M Ca(OH)2.
Ka= 4.0 x 10^-4
So I found the equivalence point = 0.02L x 1.50M HNO2/ 3.00M (cuz 2 mols of OH so 1.5 x2)= 0.01L
So i know we have to find moles of both HNO3 and Ca(OH)2.
moles of HNO3 = 0.03 mol
I'm having trouble finding the moles for Ca(OH)2 . Since the question said after an additional 2ml after equivalence point the volume will be (eq volume + 2ml) = 0.012L? So when finding moles of Ca(OH)2 do I do (0.012x3.00M)?
I don't see any number given for the molarity of HNO2 initially. From your work I think you've used 0.015 M for HNO2 but the way I read the problem that 0.015 M is the concentration of the Ca(OH)2. I don't think you can determine the equivalence point without knowing the concentration of the HNO2. Let me pick a convenient number and you can follow my calculations with whatever the correct value is in your problem. I will use 0.015M for HNO2 so the calculation will be simplified.
Ca(OH)2 + 2HNO2 ==> Ca(NO2)2 + 2H2O
I like to work in millimoles. So 20.00 mL of 0.015 M HNO2 = 20.00 x 0.015 = 0.300 millimoles HNO2 initially. So we must have
Convert to millimoles Ca(OH)2 = 0.300 mmols HNO2 x (1 mols Ca(OH)2/2 mols HNO2) = 0.15.
Since M = mmols/mL then 0.15 = 0.015/mL so mL Ca(OH)2 = 0.150/015 = 10 mL. (The fact you arrived at the same number is a coincidence because I picked 0.015 M for the HNO2). So + 2 mL after tje eqivalence point will be 12 ml of 0.015 M Ca(OH)2. What do we have at this point? We have zero HNO2 and zero Ca(OH)2 + 2 mL x 0.015 M Ca(OH)2 [that's 0.03 mmol) and we have a volume of 20 mL + 10 mL + 2 mL = 32 mL total. Also we have 10 mmols Ca(NO2)2. The new concentration of the Ca(OH)2 after the eq point is M = mmols/mL = (2 x 0.015)/32 mL = ?. The (OH^-) will be twice that and you can calculate the pH from that. Frankly, I think such a low concentration of Ca(OH)2 after the eq point means we may have needed to take into consideration the hydrolysis of the Ca(NO2)2 salt but I didn't go into that. My assumption is that this is a beginning chemistry class and that would not be included in a beginners class. Good luck.