Molecular Mass by Chromatography, Lab Report Example

Introduction

Chromatography is used as a separation method for complex mixtures. In this experiment, an ion-exchange chromatography test was conducted. The ion-exchange chromatography is a type of liquid column chromatography that has two different classes, cation and anion exchange. This particular experiment used the cation-exchange technique. In the cation-exchange technique, the cations pass through a column of cation exchange resin and the cations displace the hydrogen ions on the resin, thereby releasing the hydrogen ions into the solution passing through the column. The cations that have a higher charge bind more tightly to the resin, as well as the smaller ions. In this experiment, the ion-exchange was used to exchange an alkali metal cation with a hydrogen ion in order to determine the molecular mass of an unknown salt of an alkali metal.

Formulas/Equations

1. Molecular Weight= grams/moles
2. Grams to milligrams = 1gram=1000mg
3. Molarity = moles/L
4. %Error Molecular Weight= (Calculated Weight-True Weight)/True Weight x 100%=

 Procedure

A sample of Amberlite IR-120 resin was placed in a 50ml beaker and covered with distilled water, stirred and poured into a burret. The unknown substance was weighed on a balance and determined to be 121.2 mg. The sample was then dissolved in 5 ml of water. 100 ml of distilled water was placed in a flask. A 250 ml beaker was placed beneath the column. The stopcock was opened a drip rate of 3 seconds was applied. Water was added to ensure the solution would not drop below the resin. The solution of the unknown was then added to a clean 250 ml flask. Distilled water was then added until 30 ml of the solution was collected. The pH of the solution was tested. 10 ml of solution was added to raise the pH from acidic to near neutral. A 50 ml buret with the standard sodium hydroxide was then titrated to the collected solution from the column to the phenolphthalein end point. The moles of base and acid used were calculated, as well as the molecular mass of the unknown.

Data and Observations

Table 1. Data Table

Calculations

1. Molecular Weight of Unknown= Mass Unknown/moles of Unknown

=0.1212g/0.84995= 0.143g/mol

1. True Molecular Weight: Li + Be= 6.941g+9.01g=15.941g/6.022=0.69g/mol
2. % Error= (Molecular Weight of Unknown-True Molecular Weight)/True Molecular Weight x100%= (.143-.69)/(.69)x100% = 79%

Discussion and Conclusion

The results of the experiment show that using ion-exchange chromatography, you are able to exchange one type of cation with another hydrogen ion. The experiment went fairly well as we were able to identify the salt in the alkali metal, however, the percent error was high indicating that there were some possible errors. As seen in Figure 1, the unknown molecular weight was much smaller than the true molecular weight. Areas of error that occurred during the experiment could have been from some bubbles noted in the resin column. It was indicated that there should be no bubbles in the resin in order to avoid channel formation. In addition, it was difficult to abstract the hydrogen ions from the column before titrating. It was noted during the experiment that some of the hydrogen ions may have spilled. Therefore, the errors may have cause the increase in the percent error.