Determination of Formula Weight

ASSIGNMENT

A bottle of unlabeled inorganic salt has been found in the laboratory. Since times are tough, we have decided to determine its identity rather throw it out. We know it must be NaCl, KCl, NaNO3, NaBr, KBr, NaI, KI, since these are the only salts we use in our laboratory. Use ion-exchange chromatography to determine the identity of the salt.

CONCEPTS

The task of the chemist often involves the analysis an unknown substance. For example, a chemist may be given a water sample from a stream and be asked to determine the types and amount of pollutants that are present. Another situation might involve a chemist determining the blood chemistry of an elderly patient suffering from Alzheimer’s Disease. These are just a few examples where a chemist is required to analyze and study an unknown and draw conclusions based on the results that he or she obtains.

There are many methods that a chemist can use to determine the identity of an unknown substance. Often these various method are used in combination with one another. For example, spectroscopy, detailed study of physical properties, and chemical analysis (cation and anion analysis and titration) can all be useful tools in identifying unknown substances. In most cases these methods can be used to directly study a substance. However, in other cases, it can be useful or even necessary to convert a compound into a different species in order to use these methods more efficiently.

One class of compounds which can be altered in order to simplify their analysis are inorganic salts. Inorganic salts can be quantitatively studied by titration through their conversion into acids or bases using ion-exchange resin. A commonly encountered example of the conversion of a salt using an ion-exchange resin is found in residential water softeners, Figure 1. In this process, water is softened by replacing the hard-water cations (Mg2+, Ca2+, Fe2+) with the water softened sodium ions. In a similar fashion, ion-exchange chromatography can be used to convert a known quantity of an unknown salt into an acid or a base.

 

Figure 1.

There are two general principles involved in ion-exchange chromatography. These include the mobile phase and the stationary phase. In cation-exchange chromatography, the stationary phase, which consists of a large quantity of acid groups attached to a polymeric resin, is slurried with water and applied to a column. The mobile phase, which contains the inorganic salt dissolved in a suitable solvent, is applied to the column. As the mobile phase passes through the column, exchange between the H+ ions on the polymeric ion-exchange resin of the stationary phase and the cations of the salt in the mobile phase occur. The solution which is collected at the bottom of the column contains the acid form of the inorganic salt. The newly formed acidic solution can then titrated using a standardized base to determine the number of moles present in the sample. The molecular weight of the substance and the identity of the unknown can than be determined based on the number of moles and the weight of the sample.

 

CALCULATIONS

As you will soon see in the technique section of this laboratory, this experiment requires that you weigh a certain mass of the unknown salt, dissolve it into a solvent, and pass it over a column of ion-exchange resin. As mentioned previously, this process serves the purpose of converting the salt into its corresponding acid. For example, KCl, LiCl, and NaCl would all be transformed to HCl. Once the salt has been converted, the acid solution which is a result of this procedure is titrated using a standardized solution of sodium hydroxide. With the information obtained from your titration, you can calculate the number of moles of NaOH used.

 
Figure 2.

TECHNIQUES

PROCEDURE A (Refer to Figure 2)
  1. Weigh out 0.1000-0.1500 g of your unknown salt and dissolve it in 10 mL of distilled water.

  2. Using your 25 mL graduated cylinder, measure 15 mL of dry Amberlite IR-120 (+) a strongly acidic cation-exchange resin in the H+ form.

  3. With the stopcock closed, insert a piece of glass wool into the bottom of your column and cover it with water. Be certain that there are no air bubbles. Air bubbles can be removed by slightly tapping the side of the column.

  4. Add water to the dry resin present in your graduated cylinder until the resin is completely covered. Pour the resin and water from the graduated cylinder into the column. Again be certain that there are no air bubbles present in the column.

  5. Once the column has been prepared, add enough water to the column so that the water level rises approximately one inch above the level of the resin.

  6. With the stopcock open to allow a slow drip rate from the bottom of the column (approx. 1 drop per 2-3 sec), begin to add water in a circular motion via Pasteur pipet to the inside walls at the top of the column. Do not allow the addition of the water to be vigorous enough that the top of the resin bed is disturbed, or water addition too slow as to allow the water level to go below the level of the resin bed.

  7. Continue to add and collect water from the column until the eluting from the bottom of the column is colorless. Then add your unknown to the top of the column and begin to collect the solution from the bottom of the column with a clean 125 mL flask.

  8. When you have finished the addition of your unknown to the column, wash the flask which contained the unknown two times with 2 mL of deionized water and add each of these washing to the column.

  9. Continue to add water dropwise to the column while allowing the elutant to collect in the 125 mL flask. When 30 mL of elutant have been collected, check the pH of an elutant drop from the column using pH paper. If the pH is nearly neutral, then stop the elution and continue with the titration in procedure B. If the elutant is still acidic, collect an additional 10 mL of elutant and check the pH. Continue to collect elutant in this fashion until the elutant is nearly neutral than continue with the titration in procedure B.  

PROCEDURE B
  1.  Add 2-4 drops of the phenolphthalein indicator to the solution obtained in Procedure A.

  2. Take this solution to the burets which are located in various locations in the laboratory. Fill the buret with the 0.1000 M NaOH standardized solution present at the burets.

  3. Record the initial reading on the buret on your data sheet and begin your titration.

    Note* The amount of NaOH necessary to reach the endpoint can range anywhere from 5-40 mL. Once the endpoint is reached, record the final buret reading and calculate the molecular weight of your unknown.