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Lab: Stoichiometry Inquiry

Parts of the text on this lab will not print out. This is by design. The parts that won’t print are notes for teachers. Students don’t need those notes and they are automatically excised from the printout.

Objective

In this lab students will demonstrate how stoichiometry can be applied to a chemical reaction that produces a gas.

Overview

It is well known that when an acid reacts with a compound containing the carbonate ion, carbon dioxide is produced. In the eighteenth century Joseph Black discovered that a gas that was different from air was produced by the action of an acid on magnesium carbonate (MgCO3, or magnesia alba as he knew it). In modern symbols this reaction is given below:

MgCO3(s) + 2HCl(aq) Arrowsngl MgCl2(aq) + CO2(g) + H2O(l)

Other chemicals will react with an acid in the same way. These chemicals are known collectively as carbonates. Here are a few examples of carbonates: caclium carbonate (a.k.a. chalk), sodium carbonate (a.k.a. washing soda), and sodium bicarbonate (a.k.a. baking soda). In this lab you will write and balance an equation for the reaction of sodium carbonate with hydrochloric acid. The products will be the same (with the exception of the ionic compound that forms) as for the reaction shown above.

In the chemical equation above the notation (aq) means ‘aqueous’, that is, dissolved in water. The notation (s) means solid and (g) means gas. These notations are included to show that a gas forms in the reaction. A gas will bubble up and out of the reaction mixture and cause it to have a smaller total mass than it did before the reaction.

Stoichiometry is the technique of using the molar ratios in balanced chemical equations to calculate the amount of reactants or products. In this lab you will take a known amount of hydrochloric acid and react it with excess sodium carbonate. By using molar ratios from the balanced chemical equation you will be able to predict the amount of carbon dioxide that will form. Then you will perform the reaction in the lab in such a way as to measure how much carbon dioxide is produced. If all goes well then the numbers should be very close to one another. Specifically, you should find that your measurement is within a half a gram or less of your prediction.

Sodium carbonate is hygroscopic and therefore is best suited to being an excess reactant in this lab.

The chemicals used in this lab were chosen in order to have low to zero toxicity so that disposal and student safety will not be a problem. Safety notes are given below and are visible to students.

It is difficult to make a hydrochloric acid solution have exactly the intended concentration when performing the dilution in your own lab. If 2.0 M HCl is purchased then it is probably safe to assume that the concentration is accurate. In my lab I performed this procedure with an exact amount of sodium carbonate and found that my HCl was only 1.94 M. For this reason the amount of HCl is specified in grams for 100 mL of solution for students to use in their calculations. If you are using this lab then you may want to adjust your materials list appropriately.


Materials

  1. 100 mL hydrochloric acid (HCl) solution (~2.0 M)
  2. ~15 g Sodium carbonate monohydrate (Na2CO3·H2O)
    (a.k.a. washing soda)
  3. Lab Balance
  4. 1 250-mL erlenmeyer flask
  5. 100-mL graduated cylinder

  1. watch glass
  2. metal scoop
  3. plastic weighing dish

Safety

Procedure

You will work with one partner on your plans. Then you will get together with another pair of students to carry out the reaction and perform the experiment. The data will be gathered together by your teacher so that the class will be able to produce a total of four or five trials total.

Gather the items in the materials list. You will have to use appropriate equipment to measure the sodium carbonate and the hydrochloric acid.

In your lab notebook write and balance the chemical equation that describes the reaction between sodium carbonate and hydrochloric acid. Use the equation written in the Overview as a model and be sure you have correct chemical formulas before you balance the equation. Compare your work with your lab partner’s.

The 100 mL of hydrochloric acid solution, if measured very precisely, contains 7.07 g of HCl. In your lab notebook use this information to predict the mass of carbon dioxide that will be produced when this amount of acid reacts. Work with your lab partner to carry out this calculation correctly. Share your result and show your work to your teacher for approval before moving on.

Set up the equipment you have to carry out this chemical reaction in such as way as to be able to measure the amount of carbon dioxide produced. Discuss this with your group and come up with a plan and write it in your lab notebook. Share this written plan with your teacher for approval before carrying out your plan. Here is an important tip: Do not add all of the acid at once or it will bubble over the top of the flask and ruin your experiment.

Carry out your plan and observe the reaction closely. Record your observations in your lab notebook. Be careful to examine the flask carefully both during and after the reaction. Note changes in appearance, mass, temperature and whether or not all of the solid dissolves.

Measure the amount of carbon dioxide produced. Record it in your lab notebook and show work for the calculation that you used to measure the amount of carbon dioxide. Share the result with your teacher so the results can be shared with the rest of the class. When you have the opportunity, obtain the results of the other groups in your class and record them in your lab notebook.


Lab Report

In a typed and printed out document answer the following questions as an individual. You may not work with a partner to answer the questions and your responses will be graded based on their accuracy.

  1. Using proper subscripts and phase of matter notation, give the balanced chemical equation for the reaction you studied in this laboratory activity.
  2. In a short paragraph describe the chemical reaction you observed and how it looked when it was complete.
  3. Use the mass of hydrochloric acid given in the procedure to predict the mass of carbon dioxide that should form in your experiment. Show work for your calculation and report your answer with the correct number of signficant figures and units.
  4. When you perform the calculation in the previous question, why is it necessary to convert from grams to moles before using the ratio in the chemical equation to calculate the mass of carbon dioxide?
  5. Everyone in the class performed the same reaction with the same amount of hydrochloric acid. Ideally, everyone should have the same results. In reality, there was likely some variation. Provide a data table with the mass of carbon dioxide measured in the lab for all of the groups in your class. Calculate an average and a range for the data and include these in the data table.
  6. Is the mass you measured close to the expected mass you calculated based on stoichiometry? Compare your calculation (from question 3) to your result (from question 5). Calculate a percent difference between the two: Subtract the calculated value from the measured value and divide the result by the calculated value. Finally, multiply by 100%. If your measured value was smaller, then the percent difference will be negative. If your measure value was bigger, then the percent difference will be positive.
  7. Use stoichiometry to calculate the mass of sodium carbonate that was required to react with the 7.07 g of hydrochloric acid. Show work for your calculation.
  8. The procedure specified an amount of sodium carbonate. Did all of the sodium carbonate react? In answering this question consider both your lab observations and the result of your calculation in the previous question.
  9. If all of the sodium carbonate reacted would there have been more carbon dioxide produced, the same, or less? Explain.
  10. Why did the procedure specify an amount greater than the amount needed to react with the hydrochloric acid?
Last updated: May 24, 2016       Home