Name

Class, Period

Date

Law of Definite Composition

Introduction

Elements are a kind of matter that cannot be decomposed by ordinary chemical means. Compounds are chemical combinations of elements. What is a chemical combination as opposed to just any combination of elements? It would help to give an example of what is not a compound. The combination of oxygen and nitrogen in the air is a mixture of the gases. This mixture can be in all proportions of oxygen and nitrogen. The properties of the mixture depend on the individual properties of the individual components of the mixture. Compounds, on the other hand, are special combinations of the elements. A compound has its own properties, distinct from the properties of its elements.

The law of definite composition states that the elements forming a compound always combine in the same proportion by mass. The compound water, H2O, is always a chemical combination of hydrogen and oxygen in a 1:8 ratio by mass. If a mixture of hydrogen and oxygen were reacted, in some other mass ratio, for example 1:2, water would be formed but some hydrogen would remain unreacted. Water only forms in the 1:8 ratio by mass.

In this experiment you will examine the reaction between magnesium metal, Mg, and oxygen, O2. When heated strongly in an open crucible for several minutes. You will measure the mass of magnesium that reacts and the mass of the magnesium oxide that is formed, and use your data to calculate the mass of oxygen that combined. You can than find the ratio of mass of magnesium to the mass of oxygen and compare your experimental ratio with the ratios of your classmates and with the actual ratio. By calculating the percent error you can then evaluate how accurately you performed the experiment and look for the source of error.

Pre-Lab Discussion

Read the entire laboratory investigation and the relevant pages of your textbook. Than answer the questions that follow.

  1. Why is it so important to begin the experiment with a clean and dry crucible?

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  1. What is the purpose of making sure the outside of the magnesium ribbon is clean and shiny?

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  1. With the element or elements dose the magnesium combine when it is heated in the crucible?

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  1. In the procedure, you are asked to reheat the crucible repeatedly until the last two masses agree within the .03 g. What is the purpose of this reheating?

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  1. If you were to combine 80 g of oxygen with some hydrogen, how much hydrogen would be needed to completely use up all the oxygen?

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  1. Suppose a compound of sodium and chlorine is formed in the ratio of 1.54 g of chlorine for each gram of sodium. How much sodium would you need to completely react 45.0 g of chlorine? [Hint: (Xg Cl)(1g Na/1.54g Cl)=Yg Na]

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  1. In your own words, write the purpose of this experiment.

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Objectives

  • Observe a reaction between magnesium and oxygen
  • Calculate a ratio of mass to magnesium to mass of oxygen
  • Measure masses carefully to obtain accurate results

Materials

chemical splash goggles
laboratory apron
Crucible lid
Crucible tongs
Medicine droppers
Centigram balance
Magnesium ribbon / Pipe stem triangle
Ring stand and ring
Lab burner
Wire gauze
Sandpaper
Distilled water

Safety

Wear your goggles, gloves, and lab apron at all time during the investigation. Sodium carbonate is irritating to the skin. Do not let this material come into contact with your skin.

Procedure

  1. Obtain a piece of magnesium ribbon from your teacher. If the surface is not shiny, use a piece of sandpaper to clean the surface.
  2. Obtain a clean, dry, crucible and cover. Find the mass of the crucible and cover and record it.
  3. Roll the magnesium in a loose coil and place it into the crucible. Find the mass of the crucible, cover, and magnesium. Record it.
  4. Set up ring stand, ring, burner, and pipe stem triangle as shown in figure A. Place the crucible on the triangle. Begin heating the crucible gradually with the lid completely on. Heat slowly by moving the flame around underneath the crucible. Remove the heat temporarily if a large amount of smoke comes out of the crucible.
  5. After about four minutes of direct heating with no smoke, remove the lid slightly. Heat the crucible to redness for four minutes. Finally, remove the lid completely and heat strongly for four more minutes.
  6. Turn off the lab burner and put the lid back on the crucible. Allow the crucible and cover to cool to a temperature low enough that you can touch the crucible. Find the mass of the crucible, contents and cover. Record it.
  7. Add 10 drops of distilled water. Smell cautiously, noting any odor. Set up the crucible for heating again. Reheat for four minutes with the lid on and then let it cool once again.
  8. Find the mass of the crucible, lid, and product. Record it.
  9. Compare the masses found in steps 5 and 8. If the masses do not agree within 0.03g, reheat the crucible for four minutes, cool, and re-mass. Repeat until the last two masses agree within this range.
  10. Clean up all materials and wash your hand.

Observations

Mass of crucible and cover
Mass of crucible, cover, and magnesium
1st Mass of crucible, cover, and product
2nd Mass of crucible, cover, and product

Calculations:

  1. Find the mass of the magnesium that reacted.

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  1. Find the mass of magnesium oxide that was produced.

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  1. Find the mass of oxygen reacted.

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  1. Find the ratio of the mass of magnesium to the mass of oxygen.

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  1. The accepted ratio for the mass of magnesium to the mass of oxygen is 1.52. Calculate your percent error using the formula:

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Mass of magnesium that reacted
Mass of magnesium oxide produced
Ratio of magnesium oxide produced
Percent Error

Analysis and Conclusions

  1. Check with other members of your class to see how your results compare. Do you see any trends?

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  1. How would your results be affected if all of the magnesium did not react?

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Synthesis:

  1. Use your textbook to determine the formula for the magnesium oxide formed in the experiment.

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  1. Use the accepted ratio to determine what mass of magnesium would combine exactly with 16.0g of oxygen..

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  1. Suppose you tried to combine 42 grams of magnesium with 45 grams of oxygen. Which of the two substances would be left after the reaction? How much magnesium oxide will be formed?

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  1. When one pound of gasoline (made up of compounds of hydrogen and carbon) is burned in an automobile approximately 3 pounds of carbon dioxide, CO2, is given off. Carbon dioxide is one of the gases contributing to global warming. What information from this experiment helps explain how one pound of gasoline can give off approximately 3 times an much CO2?

(Type response here)