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Experiment 3: EMPIRICAL FORMULA OF AN OXIDE OF TIN

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    191263

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    Lansing Community College CHEM161-General Chemistry I Laboratory

    Learning Objectives

    • To determine the empirical formula of an oxide of tin by calculating the ratio of the number of moles of oxygen that combine with a sample of pure tin foil when it is dissolved and heated in concentrated nitric acid.

    INTRODUCTION

    According to the Law of Definite Proportions, in any given chemical compound the constituent elements are always combined in the same proportions by mass. The simplest integer ratio of the relative number of atoms of each element in a compound is called the empirical formula. The empirical formula should not be confused with the molecular formula of a compound, which gives the total number of atoms of each element in a compound.

    Example: Empirical formula of blood sugar (glucose):

    Molecular formula for glucose:

    Empirical formula of a compound can be determined experimentally by first finding the mass of each element present in that compound. Then converting mass of each element into moles of those elements. The resulting mole ratio of the elements when simplified to small whole numbers gives the empirical formula of the compound.

    An example to determine an empirical formula is the formation of a compound composed of lead ions and iodide ions from a reaction of elemental lead with sodium iodide. A 0.150 g sample of lead, was dissolved in 10.0 mL of 3 M nitric acid. An excess of a sodium iodide solution was added to the dissolved lead ions. A yellow precipitate was was formed in this reaction. See the table below for the data collected and the calculations in determining the empirical formula of lead iodide from this experiment.

    A

    Mass of empty beaker

    26.975 g

    B

    Mass of lead + beaker

    27.125 g

    C

    Mass of lead iodide formed + beaker

    27.492 g

    D

    Mass of lead (B-A)

    0.150 g

    E

    Mass of lead iodide (C-A)

    0.517 g

    F

    Mass of iodine (E-D)

    0.367 g

    G

    Moles of Pb 

    0.000724 mol

    H

    Moles of I

    0.00289 mol

    J

    Mole ratio of

     = 3.99

    K

    Empirical formula

    PbI4

    The mole ratio of is 4:1, giving the empirical formula of the compound, .

    This experiment involves dissolving a known mass of tin in nitric acid, . is the source of oxygen to obtain the oxide of tin. The other two products are water and orange brown nitrogen dioxide gas.

    As the tin-nitric acid mixture is heated, , and water are removed leaving the oxide of tin as the only product. The difference in mass between the original tin and the oxide of tin is the mass of oxygen in the compound. Then these masses will be used to calculate the number of moles of each element that combined to form the oxide of tin. Finally the ratio of moles of each element will allow us to write a chemical formula having the simplest whole number ratio of combining elements – the empirical formula.

    NOTE: There are several possible oxides of tin.

    AVERAGE DEVIATION AND PRECISION

    After determining the empirical formula of the oxide of tin, you will discuss the precision of your experimental value with the average of experimental value including three other students.

    Precision is defined as the closeness of a series of measurements to one another. Precision is really a measure of the ability to reproduce a result. Although good precision can be an indication of good accuracy, it is entirely possible to obtain good precision but poor accuracy and vice-versa. While there are several methods for reporting the precision, we will use the average deviation that is the sum of the absolute differences between each measured value (M) and the average value (x̅) divided by the number of measured values (n). See the Appendix for more about precision and accuracy.

     Average deviation =

    This experiment has a one-hour period between the start of the reaction between tin and nitric acid and the weighing the dry oxide of tin. For this reason, your instructor will go over the procedure and safety precautions at the beginning of the class. At this point you will start the reaction. Then you will return to seat for the rest of the prelab lecture.

     

     

    EXPERIMENTAL PROCEDURE

    Obtain a clean dry 50 mL beaker. Mark your beaker in pencil on the frosted circle. Mass the beaker. Record the mass of the empty beaker in DATA TABLE I. Do not tare the balance. Place a piece of tin in the beaker and mass the beaker with tin. Record the mass of the beaker plus tin in DATA TABLE I. Push the tin foil so that it lays flat on the bottom of the beaker.

    Bring your beaker with the flattened tin foil sample to the hood. Your instructor will dispense 6.0 mL of concentrated nitric acid into your beaker. Observe the signs of a reaction between tin and concentrated nitric acid. Record your observations on page 6.

     

    Note: Nitric acid is a strong acid and could cause a serious burn. If it comes in contact with your skin, wash the affected area immediately and then report it to your instructor. If you spill nitric acid, ask the lab aide or your instructor to help you clean it up.

    Once the reaction subsides, your instructor will place the tin-nitric acid mixture on a hot plate set at 100°C for 30 minutes and then slowly increase the temperature to about 225°C for 10 minutes and 300°C for the next 20 minutes.

    During the one hour wait, your instructor will finish the prelab lecture and give the quiz. Use the remaining time to complete the Excel Graphing Exercise using the data on page 8, which is part of this experiment.

    When no further fumes or moisture on the inside walls of the beaker are observed, obtain a wire gauze and remove the hot beaker from the hood using beaker tongs. Carry the beaker to the lab bench on the wire gauze while still holding it with the beaker tongs. Allow the beaker to cool to room temperature. When completely cooled, mass the beaker and the residue. This residue is the oxide of tin. Record the mass of beaker and the residue in DATA TABLE I.

    Following the steps on the data sheet, calculate the empirical formula for the oxide of tin you have just made. Show your calculations in the space provided under DATA TABLE I. Compare your data with the data collected by students near your lab bench and complete TABLE II.

    Clean up & Waste Disposal

    1. Discard the white solid in the labeled waste container in the hood.
    2. Wash your beaker with a brush and soap.
    3. Shut down and return the college computer to the Computer Wall Station. Be sure to plug it into the corresponding numbered spot.
    4. Wipe the benchtop with a moist paper towel.

     

     

    DATA SHEET FOR EXPERIMENT 3 - EMPIRICAL FORMULA OF AN OXIDE OF TIN

     

    Name: ___________________________ Date: ________________

    DATA TABLE I (2 pts.)

    (Record all digits from the balance retaining the correct number of significant figures.)

    A

    Mass of the beaker

    		  

    B

    Mass of the beaker & tin

    		  

    C

    Mass of the beaker & the oxide of tin (residue)

    		  

    D

    Mass of tin

    		  

    E

    Mass of the oxide of tin

    		  

    F

    Mass of oxygen atoms (in E above)

    		  

    G

    Moles of tin

    		  

    H

    Moles of oxygen atoms

    		  

    I

     Mole ratio of in the oxide of tin

    		  

    J

    Empirical formula of the oxide of tin

    		  

    CALCULATIONS (3 pts.)

    Show your calculations below. Be sure to include units in your set up and report your answer to the correct number of significant figures.

    1. Mass of tin:
    1. Mass of the oxide of tin:
    1. Mass of oxygen:
    1. Moles of tin:
    1. Moles of oxygen atoms:
    1. Mole ratio of in the oxide of tin (Do not round off by more than 0.2):

     

     

    OBSERVATIONS (2 pts.)

    Use the step numbers from your procedure followed by your observations. Use complete sentences. Include 4 observations for full credit. When applicable, more than one observation can be written for the same step.

    TABLE II (2 pts.)

     

    Your Data

    Student #1

    Student #2

    Student #3

     Mass of tin from Table I (D)

    		        

     Mass of the oxide of tin (E)

    		        

    % tin in the oxide of tin

    		        

    *Students #1, #2 and #3 are students that share your table.

    Average % tin in the oxide of tin = _________________________

    Average Deviation of % tin in the oxide of tin = ________________________

    CALCULATIONS (Each step 1 pt.)

    Show your set up for the following calculations in the space below.

    • % tin in the oxide of tin for your data:
    • Average % tin in the oxide of tin:
    • Average Deviation of % tin in the oxide of tin:

     

     

    POST LABORATORY for EXPERIMENT 3 - EMPIRICAL FORMULA OF AN OXIDE OF TIN

    Name: ____________________________________

    1. Using the average deviation from Table II, discuss the precision of the % tin in oxide of tin. Begin with the definition of precision. (A difference of less than 1.0% in the average deviation is considered good for this experiment.) (2 pts.)
    1. Was this experiment a chemical change or a physical change? Please support your conclusion with definitions and observations. (Refer to Experiment 1 if necessary.) (2 pts.)
    1. If the residue in the beaker was not heated long enough, would the actual mass of dry oxide of tin be lower or higher than the mass of the oxide of tin not dried completely? (1 pt.)
    1. If the residue in the beaker was not heated long enough, would the value entered for the mass of oxygen (F in DATA TABLE I) in wet oxide of tin be higher or lower? Explain. (1 pt.)
    1. Using your answer from question 4, would the calculated value of O/Sn mole ratio be higher or lower in wet oxide of tin? Explain. (2 pts.)

     (Over)

    1. Graphing Exercise using Excel – Submit in the Dropbox labelled Exp. 3 Volume-Mass Graph found in Exp. 3 Folder. (2 pts.)

    One of the most important ways that a chemist can show data to other people is by graphing it. As you organize and analyze data, you ask meaningful questions and dig deeper to solve problems. Moving beyond simply memorizing facts, you acquire the skills of reasoning, inquiry and communication. Building data literacy makes for a richer, more meaningful experience.

    Several cylinders of pure copper were weighed and placed into a graduated cylinder filled with water. The volume of water displaced by the copper cylinders were then measured. The results are tabulated below. Show the relationship between the volume of the cylinders (x-axis) and their mass (y-axis) using a line graph.

     

    Table 1. Volume and Mass of Copper Cylinders

    Volume of the cylinder

    Mass of the cylinder

    4.25

    37.91

    5.83

    52.00

    6.50

    57.98

    7.14

    63.69

    8.56

    76.36

    9.12

    81.35

    10.40

    92.77

    11.28

    100.62

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    Experiment 3: EMPIRICAL FORMULA OF AN OXIDE OF TIN is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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