2507 Stoichiometry of Reactions

STOICHIOMETRY OF REACTIONS

1.0 INTRODUCTION

Stoichiometry describes the ratio relationships between chemicals. These relationships are calculated mathematically using dimensional analysis by including units with quantities and canceling identical units to arrive at the unit of the answer. For example, grams can be converted to moles using the molar mass. If you were to weigh out 5.0 grams of calcium (which has chemical symbol Ca), it is possible to convert it to moles using the molar mass found on the periodic table.

(5.0 g Ca / 1) X (1.000 mol Ca / 40.08 g Ca) = 0.12 mol Ca

The units of 'grams of calcium' are in the numerator of the first term, and in the denominator of the second term. Those units cancel, and the remaining units are moles of calcium.

(5.0 g Ca / 1) X (1.000 mol Ca / 40.08 g Ca) = 0.12 mol Ca

The result should have an appropriate number of significant figures. The molar mass is defined as the number of grams of a compound for exactly 1 mole of the substance. The molar mass therefore has the same number of significant figures in the numerator as the denominator. In the above example, the molar mass has four significant figures, but the quantity of 5.0 grams of calcium only has two significant figures. Thus there should only be two significant figures in the result.

The conversion between mol and the number of atoms or molecules can be done by using Avogadro’s number (6.02 x 1023 molecules (or atoms) per mol). When a balanced chemical equation has been established, conversion between mol of one substance and mol of another is done using the coefficients in the balanced equation.

For example, the balanced equation between sulfuric acid, H2SO4 and sodium hydroxide, NaOH is:

H2SO4 + 2 NaOH → Na2SO4 + 2 H2O

This balanced equation indicates that 1 mol of H2SO4 reacts with 2 mol of NaOH to produce 1 mol of sodium sulfate, Na2SO4 and 2 mol of water, H2O.

If there is 24.0 g of H2SO4, we can determine how much NaOH is required. We will have to convert first to moles sulfuric acid using the molar mass of sulfuric acid, then to moles of sodium hydroxide using the coefficients in the balanced equation.

When setting up dimensional analysis problems, the units should be identified and retained in enough detail to avoid confusion. This is particularly important when using diluted solutions 15 ml of a solution of 1.5 molar NaOH should be written as "15 ml 1.5 M NaOH solution" to avoid confusion such as 'mol NaOH'. Similarly, "100 ml of 25% w/w H2SO4 solution" is different from "100 ml of 100% w/w H2SO4 solution"--both of these solutions have different densities which can easily lead to errors.

Figure 1 summarizes some stoichiometric relationships. These relationships are investigated in detail in this lab for reactions of calcium, water, hydrochloric acid and sodium hydroxide. Stoichiometric relationships can be reported graphically or numerically.

Figure 1 Stoichiometric Relationships

References and further reading

Los Medanos College Chemistry 25-26 Laboratory Manual, Technique B Significant Figures

Los Medanos College Chemistry 25-26 Laboratory Manual, Technique E, Volumetric Transfer of Reagent

Los Medanos College Chemistry 25-26 Laboratory Manual, Technique G, Buret Use

2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

!!Wear your safety goggles!! Sodium hydroxide and hydrochloric acid are both strong eye irritants. Be sure that the pathway to the eye wash station remains always unobstructed. If any solution of acid or base used in this experiment is spilled on your hands, rinse thoroughly with plenty of water. Wash your hands before leaving the laboratory. Do not eat or drink during this experiment. Dispose of liquids down the drain Once the reaction has been completed, the products are non-toxic. There are no hazardous wastes to be captured at the completion of this experiment.

3.0 CHEMICALS AND SolutionS

Chemical	Concentration	Approximate amount	Notes
HCl, hydrochloric acid	Approximately 1 M	50 ml	Record the concentration of the solution to 3 significant figures.
NaOH, sodium hydroxide solution	Approximately 0.5 M	100 ml	Record the concentration of the solution to 3 significant figures.
Methyl orange indicator solution	0.5%	Drops	Indicator
Calcium metal	N/A	0.2 g pieces	Calcium will react with moisture and the atmosphere. Keep stock containers sealed.
Laboratory water	N/A
Litmus/pH paper	1-2 strips	pH measurement
Matches

4.0 GLASSWARE AND APPARATUS

Item	Use	Notes
1 small test tube	Mixing
250 ml glass beaker
400 ml glass beaker	Capturing waste for final disposal
50 or 100 ml beakers	Weighing containers, solution containers	Start with clean, dry containers.
Spatula		Be sure to clean residues off spatula
Balance	Weighing
2 flasks, 125-ml or 250-ml	Titration
25-mL Pipette and bulb	Transfer liquid
50-mL Buret	Transfer liquid

5.0 PROCEDURE

Part A: Combining Calcium and Water

1. Fill a 250-mL beaker ½ full of water.
2. Obtain 1 small piece of calcium. Observe the appearance of calcium metal.
3. Record your observations in the data section.
4. Obtain a small test tube, fill with water, cover its mouth with your finger, and invert the filled test tube into the beaker. Remove your finger when the mouth of the test tube is below the water level in the beaker. The objective is to invert the test tube without having any air bubbles.
5. Drop the small piece of calcium into the beaker.
6. Move the test tube so that it is directly over the piece of calcium. Capture gas formed (if any).
7. Record your observations.
8. Test the resulting solution with blue and red litmus paper, or pH paper.
9. Record your observations in the data section.
10. Use your observations to predict the reaction between calcium and water. Balance the reaction using correct chemical formulas.
11. If gas was formed and captured, test the gas with a lit match.
12. Discard the contents of the test tube(s) as directed by your instructor, clean all glassware used so far.

Part B:

1. Weigh out between 0.05 and 0.20 grams of calcium turnings. Record the mass in the data section to the nearest 0.0001 g on the analytical balance.
2. Transfer the calcium to a 125-mL flask.
3. Repeat for a second sample.
4. Return to your lab station with your two flasks.
5. Add 25.00 mL of 1.0 M HCl with the 25-mL pipette. Review Technique E on how to do this procedure with precision. Record the exact concentration of HCl.
6. Swirl the flask until all evidence of a reaction has ceased.
7. Add 1 drop of methyl orange to the test tube. Record the color in the data section.

a. A reddish orange color indicates the presence of acid (H+).

b. Yellow indicates a more neutral solution.

c. You may need to put white paper under your beaker to clearly distinguish the color.

8. Prepare your buret by washing once with laboratory water and once with about 15 mLs of NaOH solution. Collect the waste in a beaker labelled “waste.” Then fill your buret with NaOH solution and record the initial volume reading. Review Technique G on use of a buret. Record the exact concentration of NaOH.
9. Add the NaOH solution to your first flask slowly until one drop turns the color yellow. Record this final volume.
10. Refill your buret and titrate your second sample.
11. Calculate the number of moles of NaOH required to react with the excess reactant.
12. If there is time, do a third trial.
13. Report your results of the mass of calcium you used, the mol HCl added, and the mol NaOH added.
14. Record the class data. Make sure you calculate ALL missing data in the table. This will provide practice with stoichiometric calculations.

Last Name

First Name

Partner Name(s)

Date

6.0 DATA RECORDING SHEET

Part A
Action	Observation(s)
Inspection of piece of calcium metal.
Interaction between calcium metal and water (if any)
Test of reaction water with red litmus, blue litmus and pH paper.
Hypothesis of reaction(s) occurred
Evidence to confirm or refute hypothesis
Other observations
Notes:

Part B: Titration

Exact Concentration of HCl ____________________________________________

Exact Concentration of NaOH __________________________________________

	Trial 1	Trial 2	Trial 3 or Class Data	Class Data	Class Data
Gram of Ca
Mole Ca
Volume HCl added
Mole HCl added
Initial Volume NaOH
Final Volume NaOH
Volume NaOH used
Moles NaOH used
Moles HCl remaining in flask
Moles of HCl that reacted with Ca

7.0 CALCULATIONS AND ANALYSIS SHEET

Show your work here for calculations as needed.

Graph the moles of HCl that reacted with Ca versus the moles of Ca for your data. Attach your graph and give the equation for the line. What does this tell you about the stoichiometry of the reaction?

8.0 POST-LAB QUESTIONS AND CONCLUSIONS

1. Write out the balanced chemical reactions for part A and B.

2. Magnesium is in the same family of elements as calcium. If 0.0937 g of magnesium metal is reacted with 20.0 ml of 1.248 M hydrochloric acid solution, how many milliliters of 0.445 M sodium hydroxide solution would be expected to be needed to cause the resulting mixture to turn red litmus blue?

3. Aluminum is a Group 13 element. A sample of aluminum metal reacts with hydrochloric acid, much the same way that calcium metal does. What is the balanced chemical reaction for aluminum and hydrochloric acid?