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Finding the Molar Mass (relative atomic mass) of an element Objective To calculate the relative atomic mass (amu) or molar mass (mol) of two different elements by carrying out stoichiometrio calculations based on the reactions that are carried out and the products that we formed Discussion Early chemists measured the mass of an atom of one element relative to the assigned mass of some other element chosen as a standard reference" element. John Dalton chose oxygen (O) as the reference element. Later the element hydrogen (0) was favored because it was the lightest element known. Today, a specific isotope of the element carbon is assigned an atomic weight of 12 amu (atomic mass unit) and this is used as the reference a reaction is carried the balanced equation and stoichiometry calculations will allow an experimental value of a RAM to be found For experiment one, zinc will be used as the reference element with a RAM of 65.4 amu or 65 g/mol In this experiment an excess of copper (II) sulfate solution will be reacted with a known mass of zinc metal (zinc is the limiting reagent). This is an example of a cationic single replacement reaction in which the zinc reduces the copper ions to copper metal CuSO4 + ZA ZnSO... Cu For experiment two, copper (Cu) will be used as the reference element using the RAM that you have found in experiment 1. In this experiment an excess of copper will be reacted with silver nitrate solution. This is another example of a cationic single displacement reaction. The more active copper will easily oxidize and displace the silver as in the balanced chemical equation: 2 AONO + Cow - 2 Age Cu(NO) Caution silver nitrate solutions need to be handled carefully because they will stain the skin when exposed to light
Materials 2 x 150 ml beaker Balance zinc powder silver nitrate solution (0.50M) filter paper and funnel hotplate 10 mL graduated cylinder copper wire copper (II) sulfate solution (0.50 M) stirring rod ring clamp and stand tweezers Procedure 1. Add approximately but not more than 0.3 g of zinc powder to a clean, dry 50 mL beaker. Note the exact mass of zinc and its appearance on the data sheet. 2. Use the graduated cylinder to measure out 10.0 mL of the copper (II) sulfate solution. Note the appearance of the solution on the data sheet. Add to the beaker containing the zinc. Stir for about 3 minutes. 3. Leave until no further reaction occurs, stir to confirm that the reaction is complete Note the appearance of the solid and the solution at the end the reaction on the data sheet 4. Add 10.0 mL of silver nitrate solution from the bottle provided to a second clean, dry 50 ml beaker. Add 10 mL of distilled water to the beaker. Note the appearance of the solution on the data sheet. 5. Collect a piece of precut copper wire. Wind the wire into a loose coil. Accurately find the mass of the wire and record and note the appearance on the data sheet 6. Place the copper wire in the beaker containing the silver nitrate solution. Stir for about 3 minutes. 7. Leave until no further reaction occurs, stir to confirm that the reaction is complete. Note the appearance of the solid and the solution at the end the reaction on the data sheet. 8. For the first reaction, collect a piece of filter paper, take its mass and note on the data sheet. Set up the funnel with the folded filter paper above a 100 ml beaker. Pour the contents of the beaker into the funnel, the solid will collect in the filter paper. Clean the solid first with distilled water and then with a little acetone. 9. Carefully remove the filter paper from the funnel, unfold it and place it on a hotplate. Turn the paper carefully using tweezers so that it is heated evenly. Heat until the paper is dry and then take the mass of the paper + solid and use the mass of the paper to find the mass of the copper produced. 2 N
10. For the second experiment, collect and find the mass of a second piece of filter paper, fold and place in the funnel above a 250 ml beaker 11 Remove the remaining copper from the second 50 ml beaker using a tweezers, carefully scrape any silver from the copper back into the beaker and then wash with distilled water. Dry the copper using paper towel and then find and record the final mass of the copper 12. Find the mass of the copper that has reacted by subtracting the mass of the remaining copper form that of the original 13. Filter the silver produced in the second reaction from the solution in the second beaker. Wash the silver with a small amount of distilled water and then a small amount of acetone. Discard the filtrate (the solution collected in the 250 ml beaker) into the silver waste container" 4. Carefully remove the filter paper from the funnel, unfold it and place it on a hotplate. Turn the paper carefully using tweezers so that it is heated evenly. Heat until the paper is dry and then take the mass of the paper + solid and use the mass of the paper to find the mass of the silver produced 5. Discard the silver in the silver residue container Clean up all equipment used and put away. 3
Data and Observations 1. Describe the appearance of the copper sulfate solution and the zinc metal. (2 points) Copper solfate- light blue clear zine- fine grey powder 2. Describe the changes in appearance during the course of this reaction (2 points) solution turned dark grey and became warm and the zinc clumped up at the bottom. 3. Describe the appearance of the silver nitrate solution and the copper metal. (2 points) silver nitrate-clear white copper shiny copper cool 4. Describe the changes in appearance during the course of this reaction (2 points) Sold hon became foggy the wire was siwer and was small white crystals. After 6 minutes. The souhon osa wurquoise color 5. Mass of zinc reacted 0.29174 6. Number of moles of zinc reacted (Use 65.4 g/mol) 0.0036 mois 0.3Wy mol-0.006683 65.48 7. Number of moles of copper reacted based on the balanced equation 8. Mass of filter paper (reaction 1) 9. Mass of filter paper + copper 1.0574g 1.7064g 0.6632g 10. Mass of copper produced in reaction 1 11. Calculated molar mass of copper 12. Initial mass of copper wire 0.64909 13. Final mass of copper wire 0.53859 14. Mass of copper wire reacted 4
15. Molar mass of copper from reaction 1 16. Number of moles of copper reacted 17. Mass of filter paper Mass of silver produced 1.0654+0.3779 Съъ Papec silver 18. Mass of filter paper + silver metal 19. Mass of silver produced 0-3779 20. Number of moles of silver produced based on the balanced equation 21. Calculated molar mass of silver 22. Calculate the percentage error" in the experimental determination. Percent Error = experimental value-accepted value * 100% accepted value Accepted value for copper = 63.5 g/mol Accepted value for silver = 107.9 g/mol