Lab analitica

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  • Publicado : 17 de noviembre de 2011
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Kariana M. Fuentes


Metal ions react with electron-pair donors to form coordination compounds or complexes. The metal ions accept the pair of electrons and acts as aLewis acid. In this experiment for the magnesium titration we used erichrome black T or calmagite as a metal ion indicator and Mg2+ as our metal ion. The calmagite formed a red Mgln complex after addedto the solution of Mg2+. EDTA was used as a chelating agent for a direct titration and it first reacts with the free colorless Mg2+. When the free Mg2+ is used up, the last EDTA added before theequivalence point displaces the indicator from the red MgIn complex. The change from the red MgIn to the blue unbound In indicates the end point of the titration. The concentration of our standars solutionof EDTA was 0.0097 M. The moles of MgO present on each aliquot were 4.729E-04m, 4.709E-04m, 4.738E-04m for aliquots 1, 2 and 3 respectively and the grams of MgO present in the original sample were0.1906, 0.1898, 0.1910 from aliquots 1, 2 and 3 respectively. We also determined the individual %MgO for each trial which was 11.905, 11.856 and 11.929 for trial 1, 2 and 3 respectively and our average %MgO was 11.897. The relative standard deviation in parts per thousand from our data was 3.152.


1. What are the sources of error in this experiment?
Some sources of errors couldbe systematic errors. Instrumental errors provided by the buret or the analytical balance. Some personal errors such as estimating the position of the meniscus while measuring the 25.0 mL with thevolumetric pipet and perceiving the change in color of the indicator while titrating.

2. What would happen if tap water is used instead of distilled water to dilute your sample?
Tap watercontains many ions including magnesium which will affect the determination of MgO in the unknown sample giving a positive error.

3. Why does the rate of the reaction decrease near the end point?...
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