Molar Ratios and Mass-Action Weight Relationships in Chemical Reactions:
Aluminum to Alum.

NCI Science Enrichment Program
Chemistry

Goal: In most of regular day activities, the mass (or occasionally volume) of items is the most important piece of data in recording amounts. In chemistry, we still use mass (as a convenience), but require knowledge of the relationship between the number of atoms (or molecules) of substance in a sample, and the mass of that sample. Some atoms and molecules mass a lot, and some are fairly light -- thus, one gram of lithium will represent far more atoms of lithium, than there are atoms of uranium in one gram of uranium. In this experiment, we will explore a simple chemical reaction, the conversion of aluminum (foil!) to alum (aluminum sulfate). You will compute expected versus observed yields of alum, based on the known molecular mass of the product aluminum sulfate, and the known atomic mass of aluminum.

Method:

(1) Obtain a piece of aluminum foil having a mass of about 1.00 g, and recovered its mass to the nearest 0.01g. Place about 4.00 g of potassium hydroxide in a 250 mL beaker, and add about 50 mL of water with swirling to dissolve the hydroxide.

(2) Tear up the aluminum foil into small pieces, and add it to the beaker of hydroxide solution. Place a small piece of scrap paper with your name under the beaker in the fume hodd, and allow to stand for 15 min.

(3) While you are waiting, place 13 mL of water in a second small beaker and VERY SLOWLY ADD 12 mL of concentrated sulfuric acid. ALWAYS ADD CONCENTRATED ACIDS TO WATER, NEVER THE OTHER WAY AROUND. ADD THE ACID IN VERY SMALL PORTIONS TO THE WATER, SWIRL, AND ADD THE NEXT PORTION. Carefully, place the small beaker of sulfuric acid solution in an ice water bath, without letting the ice water get into the acid solution.

(4) After the aluminum has dissolved in the hydroxide (KOH) solution, place the 250 mL beaker into an ice water bath to cool the hot solution (do not let the ice water get into the KOH solution). Allow the black residue to settle to the bottom. The reaction you just carried out is formulated as:

2 Al + 2 KOH + 6 H2O --> 2 KAl(OH)4 + 3 H2

(5) Set up a filtration apparatus to sit over a 250 mL beaker (NOT an Erlenmeyer flask) containing the cold sulfuric acid solution. Pour the solution containing the now cool KOH plus potassium alumium tetrahydroxide through the filter, so that the liquid solution drips directly into the sulfuric acid solution. After filtration, heat the beaker containing the white precipitate until all the solid dissolves. Label the beaker (a piece of paper with your name will do), then place the beaker in a fume hood to cool until the next laboratory period.

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(6) Decant the liquid from your crystals. Wash the liquid down the drain with excess water. Rinse the crystals once carefully with cool water. Set the crystals aside to dry.

(7) Weigh a dry beaker, scrape your dry crystals of alum into the beaker, and weigh the beaker with the crystals. Record all this data, as well as the computed mass of your dry crystals. Describe the crystals in your journal.

 

Workup: You should record in your lab journal the following data:

 After recording the data, calculate the mass of alum crystals that you would expect to get from your mass of aluminum, assuming that aluminum in the limiting reagent, and that 100% reaction occurred to give alum = KAl(SO4)2 12H2O. You will need to compute the formula mass of alum to do this.

 Compute your percent yield of alum.


(example journal entry)

Examples of data to record in your journal.

 

EXPERIMENT:

Molar Ratios and Mass-Action Weight Relationships in Chemical Reactions:
Aluminum to Alum.

  

DATE: x July 1997

 GOAL: To synthesize alum and record the yield of the reaction.