Calculations, Discussion, and Analysis No \"formal\" lab report Prepare a table

Question

Calculations, Discussion, and Analysis No "formal" lab report Prepare a table of the pH measurements and use the buret readings to determine the volume (in mL) of NaoH solution added. Prepare one titration curve from your "best" data with the independent variable on the x-axis, etc. Draw a smooth curve through all the plotted points. On your titration curve, indicate the volume and pH for the equivalence point, and the quarter, half and three-quarter points. Using only the "best" data, complete the following Equivalence point. The equivalence point of the titration is the point when exactly enough OH ions have been added to react with all of the HA that was originally present. It is the point of inflection in the part of the titration curve where the pH shoots straight up or nearly so. The endpoint of the titration is the point where the indicator changes color. While the endpoint is very close to the equivalence point, they are not exactly the same. To find the exact volume of NaoH needed to reach the equivalence point, replot the small region of the titration curve around this point. Draw a tangent line that touches the steepest part of the titration curve. Find the two points where the titration curve deviates from the tangent line. The equivalence point is exactly halfway between these two points. Half-equivalence point (and quarter- and three-quarter-equivalence points, too). The half. equivalence point of the titration is the point when enough OH ions have been added to react with exactly half of the HA that was originally present. To find the exact pH at the halfway point of your titration, replot the region of the titration curve around this point, using an expanded pH scale. This allows a precise interpolation of the pH. Repeat this process for the other two points

Explanation / Answer

From the plot of acid-base titration

pH at 1/2 way to the equivalence point = 2.20

ww known,

pH = pKa of acid at 1/2 equivalence point

So,

pKa = -log[Ka] = 2.20

Ka = 6.31 x 10^-3 for the unknown acid

pH at equivalence point = 8

A basic pH, means we are dealing with a weak acid. Conjugate base formed at equivalence point hydrolyzes in solution to produce OH- ions which makes the solution basic.

concentration of unknown acid solution = moles of acid/volume of acid solution

moles of acid = moles of NaOH used to reach equivalence point

                      = molarity x volume of NaOH

so with molarity of NaOH solution and using volume = 9.20 ml (volume to rach equivalence point) one can calculate moles of NaOH and thus moles of acid present in solution.

From volume of unknown acid solution taken. we can calculate concentration of unknown acid solution taken.

molar mass of unknown acid = mass of unknown acid taken/moles of acid calculated above

So,

with mass of unknown acid taken one can find the exact molar mass of the unknown acid taken for the experiment.

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