Mg determination Lab report

Topics: Chemistry, Concentration, Mole Pages: 6 (752 words) Published: September 24, 2014

Mg2+ Determination using EDTA
Linh Nguyen
Chem 1212L – 165
Feb 08, 2014

Purpose and Procedure:
The purpose of this experiment is using Compleximetric titration and EDTA to determine the concentration of Mg2+ in solution; and also calculating the percent by mass of MgO in the unknown sample. This procedure results no significant deviations.

The procedure for the experiment is from the lab manual referenced below Laboratory Experiments for Chem 1211L & 1212L (6e), with Contributions from Georgia Perimeter Faculty, Cengage Learning, Block/McKelvy. Page 153 to 158 (Experiment Title: “Determination of Mg+ using a Complexmetric Titration”)

In this experiment, EDTA (ethylenediaminetetraacetic acid) is use as a controlled chemical to determine the concentration of Mg2+. The reason why EDTA is used is because EDTA reaction has large equilibrium constants, so it is easy to measure. In basic solution, EDTA loses 4 protons to become the conjugate base commonly abbreviated asY4- as follows:


Our indicator in this experiment is EBT( Eriochrome Black T). When we add excess of EDTA to EBT-Mg2+ solution, the later will decompose and the EDTA-Mg2+ complex will form, and the solution changes from red to blue

Data table for compleximetric titration of Mg2+ sample:
Volume used: 300 mL
Mass of Na2EDTA*2H2O

0.9565 g
Molar mass of Na2 EDTA*2H2O

372.25 g/mol
Expected M of EDTA solution

8.57 E-3 M

Standardization of EDTA Solution
Concentration of Zn2+: 0.010009M
Volume used: 10.00 mL

Trial 1
Trial 2
Trial 3
Final buret reading, EDTA
18.83 ml
37.20 ml

Initial buret reading, EDTA
0.00 ml
18.83 ml

Volume used, EDTA
18.83 ml
18.37 ml

Molarity of EDTA solution
5.312 E-3 mol/L
5.445 E-3 mol/L

Average molarity of EDTA solution:
5.379 E-3 mol/L

Determination of % MgO of Unknown:
Unknown Number: 2A

Sample Calculations:
Expected molarity:
M = ((0.9565g EDTA) / (372gNa2EDTA/mol))/(0.300L solution)=8.57E-3 M Molarity of EDTA solution:
n EDTA = (0.010L Zn2+) * (0.010002) = 1.0002 E-4 mol EDTA
Since 1:1 ratio
M EDTA = 1.0002 E-4 mol EDTA/0.01883L = 5.312 E-3 M EDTA
Determination of [Mg2+]:
n EDTA react = n Mg2+
n EDTA = 5.378 E-3 g/mol * 0.0769 L= 4.136 E-4 mol
% MgO
(4.136 E-4* 40.3 * 100)/ .1999g= 8.34%
The average molarity of the EDTA solution: 5.378 E-3 mol/L
The average % of MgO : 8.27%
Error Analysis:
The data is not really good because the actual concentration of EDTA was different from expected. But the result met the expectation because the color of the solution was reached the end point by changing from red to blue. The primary errors may occur because of misreading the buret while measuring the volume of EDTA. Another error can affect the result of the experiment was that letting the volume of EDTA excess the ending point; so that the volume of EDTA was overapply that lead error. The last source of error was that our unknown sample had high percentage in mass of Mg2+, so that we need more than 50 ml of EDTA( which was the capability of the buret) to reach the end point. We had to refill the buret while measuring; and also the large volume of solution made the color harder to recognize the end point. That also led to error. Discussion:

The experiment was successfully achieved the purpose of this lab: to determine the concentration of Mg2+ using compleximetric tritation. During the experiment, the color in the unknown solution was successfully changed from red to blue showing that EDTA decompose EBT-Mg2+ solution and form the EDTA-Mg2+ complex. By calculation, we can calculate the percent in mass of Mg2+ in unknown solution. The experiment can be modified to avoid some errors by lower the mass of unknown sample, so we don’t need to refill the buret and the small volume of EDTA is better to recognize the end point.

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