The Composition of Metals and Alloys - Faculty.rmc.edu

32. Electrum is an alloy of gold and silver. An ancient coin made of electrum was found to have adensity of 11.8 g/mL. What is the percentage of gold in the coin?B. Measure the density of a quarter (method 1)Materials 100 mL graduated cylinder a quarter electronic balance1. Weigh a quarter on an electronic balance. Note: Be sure to note the exactamount in your lab notebook.2. Determine the volume of a quarter by measuring itsdiameter and thickness. The formula for the volume of acylinder is πr 2 h.3. Calculate the density. Questions:Hint – convert your units so thevolume is g/cm 3 .Show all your calculations in yourlab notebook.Of all the numbers used incalculating the density, which hasthe smallest number of significantfigures?How many significant figures are inthis number?Your calculated density will havethe same number of significantfigures. Rewrite the density, ifnecessary to show this.4. Estimate the error as a percentage. Be sure to record all calculations inyour lab notebook.C. Measure the density of a quarter (method 2)1. Fill a 100 mL graduated cylinder to around the 95 mLmark.Note: Be sure to note the exactamount in your lab notebook.2. Record the water level in the cylinder. Hint – read from the bottom of themeniscus.3. Carefully drop the same quarter you used in the previousdensity determination into the cylinder withoutsplashing. You may want to use tweezers.Interpolate between the graduationsas best you can.Hint – If water splashes out of thecylinder, start over.Watch out for air bubbles! They willthrow your calculations off.FYEC 175Alloy Composition

44. Re-read the water level. The difference is the volume ofthe quarter.5. Use your previously measured mass to find the density.6. Pour the water out, dry the quarter, and repeat thevolume measurement twice more.7. Calculate the density for each run, then calculate theaverage of the three densities.Be sure to record all measurementsin your lab notebook.Questions:Significant figures are counted tothe last digit that stays constant orvaries only slightly. Which digit isthis?This means you should assume howmany significant figures in youranswer?Rewrite the density, if necessary,with the proper number ofsignificant figures.8. Estimate the % error from the significant figures. Be sure to record all calculations inyour lab notebook.D. Measure the density of a quarter (method 3)1. Form a group with two or three other students. Writedown the names of your partners. Compare results.2. As a group decide on yet another procedure to determinethe density, one that you expect to be more accurate thanthe method in parts B & C. Present your proposal to yourinstructor. After approval carry it out. Record theprocedure carefully enough that another group ofstudents could repeat it and check your numbers.3. Report the density and % error.QuestionIs there any reason to prefer oneperson’s results? Explain.E. Measure the density of a copper shotMaterials copper shot graduated cylinder1. Weigh a weigh boat full of dry copper shot. Do not selectpieces with dimples as they may lead to air bubbles.2. Fill the graduated cylinder about half full of water. Usingthe water displacement method, determine the volume ofthe shot.Hint – Watch out for air bubbles!They will throw your calculationsoff.3. Report the density and % actual error. Hint – density of copper is given inthe table of densities in theintroduction.FYEC 175Alloy Composition

5F. Measure the density of pre-1982 penniesMaterials 30 pre-1982 pennies graduated cylinder1. Weigh 30 pre-1982 pennies. Hint – Watch out for air bubbles!They will throw your calculationsoff. Tap the side of the cylinder ifbubbles form.2. Using the water and graduated cylinder method,determine the volume of the pennies.3. Report the density and % error.4. Based on the density you measured what is the majorcomponent in the alloy used to make old pennies?Hint – consult the table of densitiesin the introduction.G. Measure the density of post-1983 penniesMaterials 30 post-1983 pennies graduated cylinder1. Weigh 30 post-1983 pennies. Hint – Watch out for air bubbles!They will throw your calculationsoff. Tap the side of the cylinder ifbubbles form.2. Using the water displacement method, determine thevolume of the pennies.3. Report the density and % error.4. Based on the density you measured what is the majorcomponent in the alloy used to make new pennies?Hint – consult the table of densitiesin the introduction.H. Measure the density of your bronzeMaterials your bronze lump graduated cylinder1. Calculate the density of the bronze lump you made in thelast experiment using the water displacement method.2. Collect the class data on the bronze alloy composition (ascalculated in the pre-lab) and the density.3. Using Excel, or some other spread sheet, plot the densityof the bronzes as a function of the % tin.QuestionHow good is the classes data?Adapted from Chemistry in the Ancient World by Patrick Hoggard and The Chemistry of Artand Artifacts by Ruth BeestonFYEC 175Alloy Composition