Partner: Joon 16 August 2011 Determination of the Empirical Formula of Silver Oxide

Transcription

1 Partner: Joon 16 August 2011 Determination of the Empirical Formula of Silver Oxide Purpose The purpose of this lab is to find the empirical formula for silver oxide. The empirical formula can be found by decomposing silver oxide using heat, massing the silver that is left and then using stoichiometry to determine the empirical formula. It would be helpful to know the empirical formula since the empirical formula can be converted to molecular formula using molar mass. It allows chemists to identify an unknown substance so they can predict the properties of the compound. Hypothesis I think that silver oxide will turn into Silver metal and Oxygen gas thus the color will turn from black to silver since oxygen will gain enough energy to overcome the bond with the silver thus forming Oxygen gas and float off. Materials: Materials Trial 1 Trial 2 Trial 3 Silver Oxide () g g g Crucible and lid 3 crucibles Tongs 2 tongs Ring stand and clamp 3 ring stands and clamps Wire gauze with ceramic 3 gauzes Clay pipestem triangle 3 triangles Bunsen Burner 3 burners Spatula 1 spatula Timer 3 timers g precision balance 1 balance Procedure 1.) Put a clay pipestem triangle on the ring stand and clamp 2.) Put a crucible and lid on the triangle 3.) Adjust the height of the clamp so that the crucible is a little higher than the Bunsen burner 4.) Light the Bunsen burner 5.) Brush the fire against the bottom of the crucible for a minute 6.) Turn off the burner 7.) Wait for it too cool down 8.) Use a tong to move the crucible and lid to the electronic balance 9.) Measure the mass of the crucible and lid to the nearest 3 decimal places 10.) Add about 0.5g of silver oxide to the crucible 11.) Place the crucible and lid on the triangle so that there is a small opening 12.) Light the Bunsen burner 13.) Brush the fire against the bottom of the crucible for 2-3 minutes 14.) Place the Bunsen burner under then crucible and heat it for 10 minutes 15.) Maximize the flame temperature

2 16.) Wait 10 more minutes 17.) Turn off the Bunsen burner 18.) Use tongs to remove the crucible and lid from the triangle and place it on a wire gauze with ceramic 19.) Slightly cover the crucible with the lid 20.) Wait 10 minutes for the crucible to cool down 21.) Measure the mass of the crucible, lid and silver metal product that is in it 22.) Throw the silver metal in the trash bin 23.) Repeat the experiment 2 more times Results: The substance in the crucible turned from black pebbles to a white powder. Under the powder was silver metal that melted and stuck to the bottom of the crucible. The bottom of the crucible turned black temporarily before turning back to white. A gas was emitted that did not have a pleasant smell. Trial 1 Empirical Formula Table Substance Mass in grams Moles (moles) Empirical Formula Percent Composition in silver oxide (%) Crucible + lid Crucible + lid + silver oxide Crucuble + lid + silver metal Silver oxide Theoretical Silver Oxygen Actual Silver Ag 5 O Oxygen Percent Yield Silver % Oxygen 77.14% Trial 2 Empirical Formula Table Substance Mass in grams Moles (moles) Empirical Formula Percent Composition in silver oxide (%) Crucible + lid Crucible + lid + silver oxide Crucuble + lid + silver metal Silver oxide Theoretical

3 Silver Oxygen Actual Silver Oxygen Percent Yield Silver % Oxygen 94.29% Trial 3 Empirical Formula Table Substance Mass in grams Moles (moles) Empirical Formula Percent Composition in silver oxide (%) Crucible + lid Crucible + lid + silver oxide Crucuble + lid + silver metal Silver oxide Theoretical Silver Oxygen Actual Silver Oxygen Percent Yield Silver 99.79% Oxygen % 2 (s) ---> 4Ag (s) + O 2 (g) Analysis: Mass of silver oxide = (mass of crucible + lid + silver oxide) (mass of crucible + lid) Trial 1: = g Trial 2: = g Trial 3: = g Mass of silver = (mass of crucible + lid + silver metal) (mass of crucible + lid) Mass of silver equals mass of crucible, lid and silver metal minus the mass of crucible and lid. Trial 1: = g Ag

4 Trial 2: = g Ag Trial 3: = g Ag Mass of oxygen = (mass of crucible + lid + silver oxide) (mass of crucible + lid + silver metal) Mass of oxygen equals mass of crucible, lid and silver oxide minus the mass of crucible, lid and silver metal. Trial 1: = g O 2 Trial 2: = g O 2 Trial 3: = g O 2 Mass Moles = (mass in grams) / (molar mass of the compound) Turn mass to mole by dividing the mass with the molar mass of the element or compound. Trial 1: / = moles Trial 1: / = moles Ag Trial 1: / = moles O Trial 2: / = moles Trial 2: / = moles Ag Trial 2: / = moles O Trial 3: / = moles Trial 3: / = moles Ag Trial 3: / = moles O Theoretical yield in mole = (moles of silver oxide) (mole ratio) Theoretical yield in mole equals to the moles of silver oxide times by the mole ratio Trial 1: ( ) (4 / 2) = moles Ag Trial 1: ( ) (1 / 2) = moles O 2 = moles O Trial 2: ( ) (4 / 2) = moles Ag Trial 2: ( ) (1 / 2) = moles O 2 = moles O Trial 3: ( ) (4 / 2) = moles Ag

5 Trial 3: ( ) (1 / 2) = moles O 2 = moles O Theoretical yield in grams = (theoretical yield in mole) (molar mass of the compound) Theoretical yield in grams equals theoretical yield in mole times the molar mass of the compound Trial 1: ( ) = g Ag Trial 1: ( ) = g O 2 Trial 2: ( ) = g Ag Trial 2: ( ) = g O 2 Trial 3: ( ) = g Ag Trial 3: ( ) = g O 2 Finding empirical formula ratio = (moles) / (moles of the smallest number) Empirical formula is found by dividing all the moles with the moles of smallest number (this will turn the moles of the smallest number into 1) Trial 1: ( ) / ( ) = 2.6 moles Ag Trial 1: ( ) / ( ) = 1 moles O --> Ag 2.6 O --> Ag 5 O 2 Trial 2: ( ) / ( ) = 2.1 moles Ag Trial 2: ( ) / ( ) = 1 moles O --> Ag 2.1 O --> Trial 3: ( ) / ( ) = 1.9 moles Ag Trial 3: ( ) / ( ) = 1 moles O --> Ag 1.9 O --> Percent Yield = (actual yield in grams) / (theoretical yield in grams) Trial 1: / = % Ag Trial 1: / = 77.14% O Trial 2: / = % Ag Trial 2: / = 94.29% O Trial 3: / = 99.79% Ag Trial 3: / = % O

6 The hypothesis isn t quite correct. However, it isn t wrong either. By the process of heating, some oxygen molecules really did turn into oxygen gas and float off. The substances in the crucible are mostly white powder rather than the silver solid. This proves that some sort of reaction had happened although the result isn t what the hypothesis suggested it to be. The oxygen was harder to remove than what had been initially predicted thus the substance didn t turn completely into silver metal. The substance turned into white powder most likely because it had some oxygen removed, but not all so it s a mixture of silver metal and silver oxide molecules. Conclusion: The hypothesis is correct but not fully correct. It could be improve to a better one. The hypothesis doesn t fully work because the oxygen molecules are quite hard to remove. Some of the oxygen molecules are still bonded in the substance, causing the product to obtain a white color instead of a silver solid. As shown in the first trial, there s most likely some errors causing the empirical formula to be Ag 5 O 2 instead of. There are many possible reasons for this error. The most likely reason for this error was that the flame wasn t intense enough and so not all oxygen was removed from the compound. Because of that, the mass of oxygen was included in the mass of silver thus the mass of silver measured was higher than it should ve been. The results of the other trial aren t perfect, either. There could ve been many errors to the experiment causing the results to be slightly mistaken. The errors could ve been human errors such as touching the crucible after heating it. The oil from the hands could ve added to the mass of the crucible causing it to weigh heavier than it should ve been. Also, the flames might not have been intense enough which is why the oxygen doesn t get completely removed. The oil from the hands could be prevent by carefully using the tongs to move the crucible and patiently wait for the crucible to completely cool down. As for the flame intensity, in the future, more than one Bunsen burner could be use to help intensifying the flame, giving oxygen enough energy to completely detach from the silver and form oxygen gas.

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