Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Measure 3–4 grams of barium chloride dihydrate.
2. Place the barium chloride dihydrate into a vial and close the cap.
3. Using a label and marker, number the vial 1.
4. Repeat steps 1–3, adjusting the vial number until you have 14 vials labeled 1–14.

Safety Precautions

Barium chloride is toxic if swallowed and harmful if inhaled. Avoid breathing dust or fumes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Handle the hot crucible and its lid only with tongs. Do not touch the crucible with fingers or hands—remember that a hot crucible looks exactly like a cold one. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. Collect all barium chloride leftover after lab. Leftover barium chloride may be handled according to Flinn Suggested Disposal Method #27h.

Lab Hints

• Enough materials are provided in this kit for 28 students working in pairs, or for 14 groups of students. This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• During the activity, do not tell students the identity of the unknown compound.
• Students will calculate a variety of formulas using the gram atomic mass table.
• One extension to the activity is after students have completed the lab and post-lab questions, let the students know the identity of the unknown chemical (barium chloride dihydrate, BaCl₂•2H₂O). Then have students calculate how much of their initial sample was BaCl₂ and how much of the initial sample was water (H₂O). Using these numbers, students can calculate the percent error in their lab.

Teacher Tips

• This experiment works best after empirical formulas and empirical calculations are introduced in the classroom. Once students master the math of empirical formulas on paper, this is a great extension activity.
• Empirical Formula of Copper Carbonate—Student Laboratory Kit, Flinn Scientific Catalog No. AP7430, is another empirical formula lab kit. In this experiment, percent by weight of copper and carbon dioxide in copper carbonate are determined experimentally by two parts, gas evolution and a colorimetric comparison.
• The molecular formula is a multiple of the empirical formula. If a student asks, there is a “teachable moment” ready and waiting to introduce the difference between empirical formulas and molecular formulas. Molecular formulas tell us the actual number of atoms or formulas in a single molecule of a compound. In order to find the molecular formula of a compound whose empirical formula is known, the molar or molecular mass of the compound must also be known.

Answers to Prelab Questions

1. Define empirical formula.

   An empirical formula is the simplest whole number ratio in which the compound can form.

2. A hydrocarbon sample is 1.88 g carbon and 0.37 g hydrogen. What is the empirical formula of the hydrocarbon?

   (1.88 g H/1) x (1 mole C/12.01 g C) = 0.1565 mol C
   (0.37 g C/1) x (1 mole H/1.01 g H) = 0.3663 mol H
   0.3663 mol H/0.1565 mol C = 2.3 x 3 = 7
   0.1565 mol C/0.1565 mol C = 1 x 3 = 3
   To get whole number subscripts, both numbers must be multiplied by 3.
   C₃H₇

3. A compound is 57.12% carbon, 6.18% hydrogen, 9.52% nitrogen and 27.18% oxygen. What is the empirical formula for this compound?

   (57.12 g C/1) x (1 mole C/12.01 g C) = 4.756 mol C
   (6.18 g H/1) x (1 mole H/1.01 g H) = 6.119 mol H
   (9.52 g N/1) x (1 mole N/14.01 g N) = 0.6795 mol N
   (27.18 g O/1) x (1 mole O/16.00 g O) = 1.69875 mol O
   4.756 mol C/0.6795 mol N = 7 x 2 = 14
   6.119 mol H/0.6795 mol N = 9 x 2 = 18
   0.6795 mol N/0.6795 mol N = 1 x 2 = 2
   1.69875 mol O/0.6795 mol N = 2.5 x 2 = 5
   To get whole number subscripts, all numbers need to be multiplied by 2.
   C₁₄H₁₈N₂O₅

   Unknowns and Formulas
   {14104_PreLabAnswers_Table_2}

Sample Data

Data Table for Unknown #__1___

White powder, solid

Answers to Questions

Calculations for Unknown 1

1. What is the mass of D and E in your unknown sample?

   16.721g – 13.270g = 2.784g of D
   3.270g – 2.784g = 0.486g of E

2. What are the moles of D and E in your unknown sample?
   {14104_Answers_Equation_1}
3. Using the moles of D and E, determine the empirical formula of your unknown.
   {14104_Answers_Equation_3}
4. What is the empirical formula of a compound that is 39.95% carbon, 13.44% hydrogen and 46.1% nitrogen?
   {14104_Answers_Equation_5}
5. (Optional) If your instructor gives you the identity of your unknown compound, calculate the percent error from your lab, using predicted final masses.
   {14104_Answers_Equation_8}

   3.270 g – 0.4824 g = 2.788 g BaCl₂
   For BaCl₂, (2.788 – 2.784)/2.784 x 100 = 0.1437%
   For H₂O, (0.486 – 0.4824)/0.486 x 100 = 0.741%

Introduction

Empirical formulas are chemical formulas that show the simplest ratio of elements in a compound. Through experimentation and careful measurements, you can determine the empirical formula of an unknown binary compound.

Concepts

• Empirical formula
• Percent composition

Background

The composition of a chemical compound—what it is made of—can be described in several different ways. The percent composition gives the percent by mass of each element in the compound and is the simplest way experimentally to describe the composition of a substance. The elements in a given compound are always present in the same proportion by mass, regardless of the source of the compound or how it is prepared. Calcium carbonate, for example, contains calcium, carbon and oxygen. It is present in eggshells and seashells, chalk and limestone, minerals and pearls. Regardless of where the calcium carbonate originates, the mass percentage of the three elements is always the same: 40% calcium, 12% carbon and 48% oxygen.

The percent composition of a compound tells us what elements are present in the compound and their mass ratio. The empirical formula of the compound lists the elements in the compound and the simplest whole number ratio in which the compound can form. For example, the empirical formula of calcium carbonate is CaCO₃—one part calcium, one part carbon, and three parts oxygen. The empirical formula gives the ratio of atoms in a compound and does not necessarily represent the actual number of atoms in a molecule or formula unit.

In this lab, your group will be given an unknown binary compound to analyze. Each group’s compound has the general formula D_xE_y. When heated, the compound decomposes and E becomes a gas.

29.6 g S
70.4 g F

(29.6 g S/1) x (1 mole S/32.06 g S) = 0.9233 mol S
(70.4 g F/1) x (1 mole/19.00 g F) = 3.7052 mol F

0.9233 mol S/0.9233 mol S = 1
3.7052 mol F/0.9233 mol S = 4

Experiment Overview

The purpose of this investigation is to determine the empirical formula of an unknown binary compound. Each lab group will be given an unknown sample number. Using your unknown number and the gram atomic mases of D and E, your group will determine the empirical formula of your unknown (D_xE_y) by massing the sample before and after heating.

Materials

Prelab Questions

1. Define empirical formula.
2. A hydrocarbon sample is 1.88 g carbon and 0.37 g hydrogen. What is the empirical formula of the hydrocarbon?
3. A compound is 57.12% carbon, 6.18% hydrogen, 9.52% nitrogen and 27.18% oxygen. What is the empirical formula for this compound?

Safety Precautions

The unknown compounds are toxic if swallowed and harmful if inhaled. Avoid breathing dust or fumes. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Handle the hot crucible and its lid only with tongs. Do not touch the crucible with fingers or hands—remember that a hot crucible looks exactly like a cold one. Please follow all laboratory safety guidelines.

Procedure

1. Obtain a clean porcelain crucible and lid.
2. Set up a Bunsen burner and support stand set up as in Figure 1.
   {14104_Procedure_Figure_1}
3. Make sure the crucible and lid are dry. If it is wet from cleaning, dry the crucible by heating it on a clay triangle with a Bunsen burner.
4. Remove the crucible and lid from the clay triangle and allow the crucible and lid to cool on the wire gauze with ceramic center.
5. Obtain your unknown and record the unknown number on the Empirical Formula Worksheet.
6. When the crucible is cool, measure the mass of the crucible to the nearest 0.001 g. Record the mass in the data table.
7. Zero the balance with the crucible on it and then add the contents of the unknown vial into the crucible. The unknown mass should be between 3–4 g. Record the mass to the nearest 0.001 g.
8. Record what your unknown looks like under Observations on the worksheet.
9. Place the crucible and sample on the clay triangle and begin to heat slowly (see Figure 2).
   {14104_Procedure_Figure_2}
10. Increase the heat and heat the crucible strongly for 5–8 minutes to heat away the E gas.
11. Remove the crucible from the heat and allow to cool on the wire gauze with ceramic center (see Figure 3).
    {14104_Procedure_Figure_3}
12. Once cool, mass the remaining D solid and crucible. Record the mass in the data table.
13. Reheat the crucible and contents for another 5–8 minutes.
14. Remove the crucible and contents from the heat and allow to cool on the wire gauze.
15. Once cool, mass the crucible and contents. If the mass does not match the mass recorded in step 12, repeat the heating until the mass is constant.
16. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

14104_Student1.pdf