chemistry help needed! titration curves?

Question

how do you calculate Ka from a titration curve? you only know the pH and volume of NaOH added.

Answer 1

Go to the equivalence point (half way up the vertical portion). Read off the volume of NaOH. Halve this volume, and read off the pH at this volume. At this precise point (half-neutralisation) pH = pKa.

Answer 2

since its strong acid+strong base the product is H2O and salt, you know its going to be a 1:1 reaction, and you have the concentrations. Ba(OH)(aq) + HCl(aq) --> H2O(l) + BaCl(aq) *always include the equation, (aq) means aqueous, pretty much anything in solution the first point is good, the last point should be pH=7. add 3mL each point not 5, the equivalence point should be reached at 12mL HCl. 1/1.25=0.8 and 0.8*15mL=12mL, so 0ml, 3ml,6ml,9ml, and 12ml. sorry in advance for the crazy way to write it, easier on paper obviously 2nd point should go; 1.5*10^(-3)mol OH - 3.75*10^(-4)mol H+ = 1.125*10^(3-) OH /*makes a little salt and water pOH= -log(1.125*10^(3-) OH / .018L = 1.2 pH = 14 - 1.2 = 12.8 same steps for the next points; point 3 6mL HCl pOH=1.447 pH=12.5 point 4 9mL HCl pOH=1.8 pH=12.2 point 5 12mL HCl pH=pOH=7 * you might want to add a point like 11mL or graph it with .01mL increments, the reason behind this is that when titrating strong acids with strong bases the graph goes really steep through the equivalent point. you'll see this when you use a burret, like one drop and the indicator used will change, in our labs we usually phenol red i believe and its like a faint pink color your aiming for, either way good luck.