- In this experiment, you will find the molarity of a hydrochloric acid (HCl) solution by quantitative analysis of its reaction with aluminum. Aluminum reacts with hydrochloric acid to form two products. The balanced equation for this reaction is as follows:
Aluminum + Hydrochloric
acid---> Aluminum
chloride+ Hydrogen
gas2 Al + 6 HCl ---> 2 AlCl3 + 3 H2 CAUTION: Hydrogen gas is highly flammable.
MATERIALS NEEDED:
Safety goggles 
10-mL volumetric Pipette Test tube Pipette pump Test tube rack *Hydrochloric acid solution 20 cm length of #18 aluminum wire
or an aluminum nail*You can use several solutions of different molarities. Try concentrations of 3M or less. It is a good idea to run a test reaction with the strongest solution, to make sure it does not completely use the aluminum sample. You need some aluminum remaining to calculate the molarity.
PRE-LAB:
1. Record the atomic weight of aluminum, and the formula weights of both aluminum chloride and hydrogen gas (H2). Round to four significant figures.
Atomic weight of aluminum. _________
Formula weight of aluminum chloride (AlCl3). _________
Formula weight of hydrogen gas (H2). _________
PROCEDURE:
DAY 1:Safety goggles on for entire lab.
2. Obtain approximately 20 cm of #18 aluminum wire, or an aluminum nail, and bend the wire sample into fourths. Find the mass of the wire to the nearest 0.01 gram, or more accurately if you have a balance that will mass to three or four decimal places. Record the mass.
Mass of aluminum wire BEFORE REACTION. _________ g
3. Mark your test tube with the appropriate information (ie. solution number, your initials), place it in your test tube rack, and carry it to the place where the hydrochloric acid sample is to be obtained.
- Using correct laboratory techniques for pipetting, obtain a 10.0 mL sample of the hydrochloric acid solution and dispense into your marked test tube. This volume is recorded for you. If you are testing more than one acid solution, use a clean, dry pipette for each solution.
Volume of hydrochloric acid solution. 10.0 mL
4. Add the aluminum wire to the test tube with the acid sample. Be careful not to splash acid. Tip the test tube to one side, and let the wire sample slide slowly into the acid solution. Observe the reaction for a few moments. The aluminum chloride will go into solution, and the bubbles are hydrogen gas.
- Place your test tube and rack inside the fume hood. This will evacuate most of the H2 gas. Leave in the hood over-night.
DAY 2:
5. Take your test tube from the test tube rack and carefully pour off the aluminum chloride without loosing the remaining aluminum. Rinse the tube and aluminum with several small rinses of distilled water.
6. Remove the remaining aluminum and put on a paper towel to dry.
7. When the remaining aluminum from your sample is completely dry, find its mass to the same accuracy as before, and record.
Mass of aluminum wire AFTER REACTION. _________ g
CALCULATIONS:
8. Calculate the mass of aluminum that reacted. This is simply subtracting the mass after reaction from the mass before reaction.
Mass of aluminum that reacted. _________ g
9. Using the balanced equation for the reaction, and the necessary data, find the molarity of the hydrochloric acid sample. Remember, first convert grams of aluminum to moles, then from moles of aluminum to moles of hydrochloric acid (this is a 2 to 6 relationship). The last is to convert to molarity using the volume. Remember, the volume is in milliliters, but you need to convert to liters to use in calculating molarity. The unit you are left with is g/L, and that is molarity.
- The calculation would be as follows:
Molarity of hydrochloric acid solution. _______ Molar
10. Using the balanced equation for the reaction, and the necessary data, find the number of grams of both aluminum chloride, and hydrogen gas produced. Remember, first convert grams of aluminum reacted to moles, then from moles of aluminum to moles of the substance you are calculating, and then to grams.
- The calculation for aluminum chloride would be as follows:
- Notice that the relationship in the balanced equation for moles of Al and moles of AlCl3 is 2 to 2. The calculation for hydrogen gas is similar, but the relationship of moles of Al to moles of H2 is 2 to 3.
Grams of aluminum chloride produced. _________ g
Grams of hydrogen gas produced. _________ g
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- Using correct laboratory techniques for pipetting, obtain a 10.0 mL sample of the hydrochloric acid solution and dispense into your marked test tube. This volume is recorded for you. If you are testing more than one acid solution, use a clean, dry pipette for each solution.
FINDING MOLARITY OF AN HCl SOLUTION
BY QUANTITATIVE ANALYSIS
INTRODUCTION: