CHEM121 – Experiment 2

Stefan Martensson

CHEM121 – Experiment 2 – Preparation of a Standard Solution and the Analysis of an Unknown Acid.

Purpose: To use Xylenesulphonic acid (XSA) from Lab1 as a primary standard for the standardization of a sodium hydroxide solution, and use the sodium hydroxide to determine the weight of an acid that reacts with a ratio of: one mole sodium hydroxide to one mole of unknown acid.

Procedure: As in CHEM121-Lab Manual, Camosun College, Winter 2010, p.7-9.

Data and Results:

Table 1 – Results from four independent titration of NaOH to achieve an acceptable standard ratio of accuracy with each sample of XSA.

TRIAL Mass XSA Volume NaOH (ml) Ratio
Initial Final Total
1 0.5285 0.50 39.95 39.45 74.645
2 0.4688 0.00 34.89 34.89 74.424
3 0.4907 0.00 36.50 36.50 74.384
4 0.5096 0.50 38.40 37.90 74.372
Mean 0.4897 36.43 74.392

The first trial’s ratio was outside the 1% of similarity and thus omitted from any future calculations.

Concentration of NaOH: (0.4897g/186.228g/mol)/0.03643=0.07218mol/l

Unknown Acid: Marked C

Table 2 – Results from three titration trials of NaOH with an unknown acid.

Unknown Trial Weight of Sample (g) Volume NaOH (ml) Ratio
Initial Final Total
1 0.2008 0.50 39.70 39.20 195.22
2 0.2009 0.10 39.40 39.30 195.62
3 0.2003 0.00 39.10 39.10 195.21
Mean 0.2007 39.20 195.35

Calculation of the mass of the unknown acid:

0.03920l*0.07218mol/l NaOH = 0.002829mol NaOH

1.000mol/0.002829mol = 353.4248

0.2007g (Unknown)*353.4248 = 70.93236

70.9g of unknown acid to neutralize one mole of NaOH.


Discussion:

The titrations to get an acceptable standard solution are indicated in Table 1. A source of error for this exercise was that it was not a benefit to change students between each titration. To minimize slight difference in technique that could influence the end result, it is better one student do each group of titrations, or even better if one person does them all for accuracy.

The first trial to establish a standard concentration of NaOH was omitted due to it being outside of a satisfactory level of accuracy.

The findings were that the concentration of NaOH was 7.218*10-2 mol/l.

The second table, the same student did all the titrations in the series, and the accuracy was greatly improved. It was found that a mean of 0.2007g Unknown Acid ‘C’ used 39.20ml of NaOH to neutralize.

Conclusion:

The Unknown Acid ‘C’ contained 70.9g of acid per mole of NaOH.

Leave a Reply