The Formula of a Chemical Compound PDF

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General Chemistry 1100...

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Lab 4: The Formula of a Chemical Compound – Page 1

Lab 4: The Formula of a Chemical Compound

Lab 4: The Formula of a Chemical Compound – Page 2 PURPOSE To determine the empirical formula of a compound formed from copper and sulfide by method of synthesis, and their stoichiometric information. INTRODUCTION AND THEORY This lab helps understand what to do with new compounds in order to figure out their empirical formulas. One of the different ways of obtaining the empirical formula of a compound is by the process of synthesis which is what was done in this experiment. The compound was built from simpler components by using copper wire and adding sulfur to it after recording the mass of crucible and mass of crucible plus copper before adding sulfur to it. Eventually an empirical formula was obtained from the synthesis and stoichiometric information of a sulfide of copper. It is important to note that the empirical formula of a compound only provides the simplest ratio of the numbers of different atoms in the compound. This is different from the molecular formula which gives the actual number of the atoms in the compound. MATERIALS 

Crucible



Clay triangle



Bunsen burner



Ring stand



Tongs



Balance



Copper wire



Sulfur

Lab 4: The Formula of a Chemical Compound – Page 3

EXPERIMENTAL PROCEDURE See Goldwhite, H.; Tikkanen, W.; Kubo-Anderson, V.; Mathias, E. Experiments in general chemistry; Macmillan learning curriculum solutions: Plymouth, MI, 2018. RAW DATA Mass of Crucible (g)

Mass of Copper (g)

Mass of Crucible + Copper Wire (g)

Mass of Crucible + Copper Sulfide after 1st heating (g)

Mass of Crucible + Copper Sulfide after 2nd heating (g)

12.386 g=

0.4966

12.883

13.019

13.010

Moles of Copper (mol) 7.81 × 10−3

Moles of Sulfur after 1st Heating (mol) 4.23 ×10−3

Moles of Sulfur after 2nd Heating (mol) 3.96 ×10−3

RESULTS The results show that there were 2 moles of copper for 1 mole of sulfide. The empirical formula obtained was

Cu2 S , meaning that the ratio of copper to sulfur was 2:1. The molecular

formula in this case was

Cu12 S 6 . In the first heating, the mass of sulfur after 1st heating was

4.23 ×10−3 g , and the subscript for sulfur was less than 2, so it required another reheat and reweigh. In the second heating, the mass of sulfur was

3.96 ×10−3 g. The percent

compositions of copper and sulfide were obtained by using the following formula:

mass of part ×100 % mass of whole

Lab 4: The Formula of a Chemical Compound – Page 4 g Cu mol 2 ×100=79.853 % Cu Copper % compositon: g 159.157 Cu S mol 2 127.092

g mol ×100=20.147 % S Sulfur % composition: g Cu S 159.157 mol 2 32.065

DISCUSSION From the discussion questions, one of the ways the empirical formula of copper sulfide would be affected if not all of the copper wire were converted to copper sulfide would be if there were excess copper, the subscripts would increase. If all the excess sulfur was not burned off, the empirical formula of copper sulfide would be affected because then there would be excess sulfur. The formula determined Cu2S is an empirical formula because it’s in the lowest ratio. If it was supposed that the mass determined for the final piece of data, the mass of crucible plus copper sulfide after second heating was 0.002 g too low, what would be the percentage of copper calculated? In order to answer that question, the original percentage of sulfur was 20.1%. If the mass of copper sulfide after second heating would have been 0.002 g too low, then there would have been an error because there’d be a higher amount of sulfide which would affect the ratios. The uncertainty in the subscripts of the formula found in this experiment were done by using the following formula: Sulfur =

[(

)

]

[(

)

]

0.00005 g+ 0.00005 g 0.00005 g+ 0.00005 g × 2 × 100= ×2 ×100=0.1573 % mass of sulfur 0.1271 g S

Lab 4: The Formula of a Chemical Compound – Page 5 Copper =

⌊

0.000010 g g g+ 0.00005 g 0.00005 g+0.00005 g + ⌋ × 100=⌊ ( ⌋ ×100= 0.0988 ( 0.00005 )+( 0.000010 0.1271 g S ) mass of copper ) ( mass of sulfur ) 0.4966 g Cu

. Errors that could arise from this experiment would be if the masses were calculated incorrectly

or if the crucible with copper were not heated enough and allowed plenty of time to cool down, therefor affecting the empirical formula and now allowing for a formula with lowest ratios.

Literature Cited Goldwhite, H.; Tikkanen, W.; Kubo-Anderson, V.; Mathias, E. Experiments in general chemistry; Macmillan learning curriculum solutions: Plymouth, MI, 2018.

Lab 4: The Formula of a Chemical Compound – Page 6...