How to solve this chemical question? (about chemistry)

Question

It is time for the titration. You measure 15.0 mL of the H₂C₂O₄ solution with a 100-mL graduated cylinder and add it to a 125-mL Erlenmeyer flask. Then you add two drops of the indicator, phenolphthalein, to the oxalic acid solution in the flask. You dispense the NaOH solution from the buret into the flask (while swirling the flask) until you reach the endpoint. At the endpoint, the solution in the flask turns light pink. You repeat the titration two additional times for a total of three trials. The initial and final volumes of NaOH used in each trial are given to you below. Calculate the moles of H₂C₂O₄ used in the titration, the moles of NaOH used in the titration, the volume of NaOH used, the molarity of NaOH in each trial, the average molarity of the NaOH solution, and the percent error. To calculate the average molarity of the NaOH solution, average the molarities of the three trials. Show ALL calculations on a separate sheet of paper. 

H₂C₂O₄ (aq) + 2 NaOH (aq) --> Na₂C₂O₄ (aq) + 2 H₂O (l)

Molar mass of H2C2O4•2H2O 126.07 g/mol 

Mass of H2C2O4•2H2O 1.5232g 

No. of moles of H2C2O4•2H2O 0.012082 moles 

volume of H2C2O4•2H2O solution 250.0 mL 

Molarity H2C2O4 standard solution 0.04833 M

(I)       (II)        (III)

Volume of H₂C₂O₄ used  15.0 0 mL    15.0 0   mL   15.00____mL

1) Moles H₂C₂O₄ used    _______mol     ______mol      ______mol

2) Moles NaOH            _____mol    _____mol    _____mol

Initial Buret Reading    _____0.00_mL  __0.05_mL     _0.10__mL

Final Buret Reading      ____14.45mL     14.38_mL   15.40_mL

Volume NaOH used      ______mL     ______mL    ____mL

3) M NaOH____M____M____M

4) Average Molarity of NaOH      ____________M 

True Molarity of NaOH          0.1000 M

5) Percent Error                _________

How to solve this question?

Answer 1

Hi Ooyeon O.,

I'll provide some guidance, and let you work the math on your own, which you should do anyway!

Oxalic acid is a diprotic acid, that means it releases 2 H+ ions per molecule of the substance.

That should start you off well: it means that the number of moles of NaOH required to neutralize will be 2 times the number of moles of the oxalic acid (NaOH is a monoprotic base). In the text below, line numbers are as (#).

That gives you numbers to plug into the (1) and (2)a lines. The 2(d) line is obtained by the difference of the (2)c and (2)b lines -- I hope you can handle that?

(2)d and (2)a lines enable you to calculate the (3) line. M is in units of moles/L, so you need to convert 2(d) to L as you calculate!

average of (3) values is (4).

Compare (4) to the theoretical. Remember that Error is defined as how much high (+) or low (-) YOUR value is vs. the THEORETICAL. That will allow you to always assign the correct signage to it (though, I have seen occasional places where misguided non-scientists assign it in reverse!!)

There is an additional scientific consideration that should be applied when you have multiple data points, as you do here. And that is uncertainty of the result, based on the demonstrated reproducibiity of the method. In other words, if you were supposed to get the result "1" and you got, in two replicate trials, the results "0" and "2", even though the average is "1", your method (or technique!) are essentially useless -- you can't rely on the test to always give you "1", ever!. Performing replicate analyses is the standard way of detecting such indeterminate (and to a lesser power also determinate) errors. But this is beyond the stated content of your current task. So bear it in mind for the future, or to read up on a bit, if you like.

-- Cheers, -- Mr d.