- In this experiment, you will find the molarity of a nitric acid (HNO3) solution by quantitative analysis of its reaction with copper. Copper reacts with nitric acid to form three products. The balanced equation for this reaction is as follows:
Copper + Nitric
acid---> Copper (II)
nitrate+ Nitrogen
dioxide+ Water Cu + 4 HNO3 ---> Cu(NO3)2 + 2 NO2 + 2 H2O CAUTION: The test tubes should be placed in a fume hood for the reaction, to evacuate the nitrogen dioxide.
REMEMBER: Nitric acid burns the skin, and nitrogen dioxide is not pleasant to breathe.MATERIALS NEEDED:
Safety goggles 
10-mL volumetric Pipette Test tube Pipette pump Test tube rack *Nitric acid solution 20 cm length of #18 copper wire *You can use several solutions of different molarities. Try concentrations of 2M 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 copper sample. You need some copper remaining to calculate the molarity.
PRE-LAB:
1. Record the atomic weight of copper, and the formula weights of both copper (II) nitrate and nitrogen dioxide. Round to four significant figures.
Atomic weight of copper. _________
Formula weight of copper (II) nitrate. _________
Formula weight of nitrogen dioxide. _________
PROCEDURE:
DAY 1:Safety goggles on for entire lab.
2. Obtain approximately 20 cm of #18 copper wire 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 copper 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 nitric acid sample is to be obtained.
- Using correct laboratory techniques for pipetting, obtain a 10.0 mL sample of the nitric 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 nitric acid solution. 10.0 mL
4. Add the copper 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 green or blue color is the formation of copper (II) nitrate, and the brown fumes are nitrogen dioxide gas.
AVOID PROLONGED OR HEAVY EXPOSURE TO THE NITROGEN DIOXIDE.
- Place your test tube and rack inside the fume hood. This will evacuate most of the NO2 gas. Leave in the hood over-night
DAY 2:
5. Take your test tube from the test tube rack and carefully pour off the copper (II) nitrate without loosing the remaining copper. Rinse the tube and copper with several small rinses of distilled water.
6. Remove the remaining copper and put on a paper towel to dry.
7. When the remaining copper from your sample is completely dry, find its mass to the same accuracy as before, and record.
Mass of copper wire AFTER REACTION. _________ g
CALCULATIONS:
8. Calculate the mass of copper that reacted. This is simply subtracting the mass after reaction from the mass before reaction.
Mass of copper that reacted. _________ g
9. Using the balanced equation for the reaction, and the necessary data, find the molarity of the nitric acid sample. Remember, first convert grams of copper to moles, then from moles of copper to moles of nitric acid (this is a 1 to 4 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 nitric acid solution. _______ Molar
10. Using the balanced equation for the reaction, and the necessary data, find the number of grams of both copper (II) nitrate and nitrogen dioxide produced. Remember, first convert grams of copper reacted to moles, then from moles of copper to moles of the substance you are calculating, and then to grams.
- The calculation for nitrogen dioxide would be as follows:
- Notice that the relationship in the balanced equation for moles of Cu and moles of NO2 is one to two. The calculation for copper (II) nitrate is similar, but the relationship of moles of Cu to moles of Cu(NO3)2 is one to one.
Grams of nitrogen dioxide produced. _________ g
Grams of copper (II) nitrate produced. _________ g
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Back to Lab Dad's Laboratory
- Using correct laboratory techniques for pipetting, obtain a 10.0 mL sample of the nitric 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
BY QUANTITATIVE ANALYSIS
OF A REACTION
INTRODUCTION: