Determine whether the metal is magnesium, iron, or zinc based on the value of the calculated molar mass.

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Mercy Neal
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1 Gases Part A: A student working at METAL Company found an unlabelled bottle of a metal in the lab. The metal could be magnesium, iron, or zinc. Each of these metals react with dilute hydrochloric acid to produce hydrogen gas and a metal chloride. You have been asked to determine the molar mass of the unknown metal. Would finding the molar mass of the metal help in identifying whether the unknown metal is magnesium, iron, or zinc? The molar mass of the metal will be determined by reacting the metal with excess dilute hydrochloric acid to produce hydrogen gas H2 (g). The hydrogen gas is collected over H2O in an inverted buret. The volume of hydrogen gas produced is measured by reading the buret. M (s) + 2 HCl (aq) H2 (g) + MCl2 (aq) 1 mole of metal M produces one mole of H2 gas By measuring the volume of H2 gas produced, temperature, and total pressure, the moles of H2 gas produced can be calculated from the Ideal Gas Law PV = nrt The pressure of dry H2 gas can be determined with Dalton s Law of Partial Pressure. P H2 gas + P H2O = Total Pressure = Barometric Pressure P H2 gas = Barometric Pressure - P H2O Determine whether the metal is magnesium, iron, or zinc based on the value of the calculated molar mass. Part B: Alloys of aluminum are often used as engineering materials in the manufacturing of commercial aircrafts, railcars, and ships, and in beverage and food cans. Aluminum alloys are good electrical conductors. They resist corrosion and are lightweight and durable. Alloys of aluminum often contain magnesium, copper, silicon, zinc, or manganese. METAL Company has asked you to determine the percent composition of their aluminum-zinc alloy. For this alloy to be used in manufacturing, it must meet strict composition requirements so that it resists corrosion. In Part B of this experiment you will determine the percent composition of this zinc-aluminum alloy. The percent composition of an aluminum-zinc alloy will be determined by reacting an alloy of known mass with excess dilute hydrochloric acid. Both the aluminum and the zinc contained in the alloy react to produce hydrogen gas. The hydrogen gas is collected over H2O in an inverted buret. The volume of hydrogen gas produced is measured by reading the buret.

2 The aluminum in the alloy reacts with HCl to produce H2 gas: 2 Al (s) + 6 HCl (aq) 3 H2 (g) + 2 AlCl3 (aq) Since 2 mole Al produces 3 mole H2 gas, then 54.0 grams Al produces 3 mole H2 gas or 1.00 gram Al produces.0556 mole H2 gas The zinc in the alloy also reacts with HCl to produce H2 gas: Zn (s) + 2 HCl (aq) H2 (g) + ZnCl2 (aq) Since 1 mole Zn produces one mole H2 gas, then 65.4 grams Zn produces 1 mole H2 gas or 1.00 gram Zn produces.0153 mole H2 gas Therefore moles H2 = n H2 = (grams Al x.0556) + (grams Zn x.0153) For a 1.00 gram alloy sample, grams Al = mass fraction of Al or % Al / 100 grams Zn = mass fraction Zn or % Zn / 100 Moles H2 produced per gram sample = ( % Al x.0556) + ( % Zn x.0153) Substitute % Zn = % Al Moles H2 produced per gram sample = ( % Al x.0556) + [ (100 - %Al) x.0153 ] To determine the percent composition of the alloy, let the variable x represent % Al and solve the above equation for x. (Hint: Multiply both sides of the equation by 100.) Solve for x which equals % Al. Then % Zn = % Al ** By measuring the volume of H2 gas produced, temperature, and total pressure, the moles of H2 gas produced can be calculated from the Ideal Gas Law PV = nrt The pressure of dry H2 gas can be determined with Dalton s Law of Partial Pressure. P H2 gas + P H2O = Total Pressure = Barometric Pressure

3 P H2 gas = Barometric Pressure - P H2O Objectives 1. Manipulate the Ideal Gas Equation. 2. Calculate the molar mass of a gas. 3. Determine the percent composition of an aluminum-zinc alloy. Prior to lab read the sections of our textbook that address the Ideal Gas Law and calculations involving the ideal gas law. References: Chemical Principles in the Laboratory, Emil J Slowinski, Wayne C. Wolsey, Robert Rossi Experiment 10: Analysis of Aluminum Zinc alloy Chemicals: 4.0 M HCl (60-80 ml of for each trial) to 0.04 grams of unknown metal strips or alloy for each trial Caution: Keep the metal strips in the jar. The metal oxidizes with the air resulting in a dull finish, a slower reaction, and less accurate results. If the strips are not shiny, they have begun to oxidize. Equipment: small RIMLESS test tube 50.0 ml buret buret stand pipet bulb 250 ml beaker thermometer Sample Calculation: A student is asked to determine the molar mass of a compound. A g sample is placed in a 125 ml flask. The liquid is vaporized and the pressure is determined to be 756 torr at 98 o C. What is the molar mass of this compound? PV = nrt R = L atm/ mol K

4 mole 60.0 g/ mol This exercise will contain two parts. Each part has a separate page of instructions and a worksheet to enter collected data and make calculations. Part A: Determination of the molar mass of a metal. Part B: Determination of the percent composition of an alloy. Download Spreadsheet for Molar Mass of Metal Download Spreadsheet for Alloy Composition Part A: Determination of Molar Mass of a Metal Before beginning the experiment, PRACTICE this technique: Use only water in the beaker and in the rimless test tube. With a pipet bulb, raise the water level in the inverted buret to about 30 ml. Turn the stopcock and check for leaks. The water level in the inverted buret should stay constant. If there is a leak in the system, the atmospheric pressure will push the water level down. All leaks must be fixed before proceeding with the experiment. Caution: Keep the metal strips in the jar. The metal oxidizes with the air resulting in a dull finish, a slower reaction, and less accurate results. If the strips are not shiny, they have begun to oxidize. 1. Weigh out to grams of metal (smaller is better for this experiment). Record the mass to +/-0.001grams. 2. Place the metal in the rimless test tube. Fill the test tube with R.O. water to the top. 3. Turn the buret upside down and place it over the test tube containing the metal and water. Place the buret (with filled test tube) into an empty 250 ml beaker. Clamp the buret to the buret stand. Make sure to close the stockcock. 4. Pour approx ml of 4 M HCl into the 250 beaker. 5. Open the stockcock and use a pipet bulb to draw up the liquid to approx. 30 to 35mL mark. Quickly close the stockcock. Read and record the volume. (Yes, read the buret upside down). Watch the reaction begin. Gas bubbles should begin to form. 6. When gas bubbles are no longer forming, read the volume on the buret and record it as the final volume. (Yes, read the buret upside down). 7. Measure the temperature of the HCl in the beaker. 8. Dispose of any residual metal in the waste container.

5 9. Repeat this procedure 2 more times and record data. Your instructor will provide you with the barometric pressure and vapor pressure of water. Part B: Procedure for Determination of Percent Composition of an Alloy: 1. Weigh out about 0.02 or less grams of the Zn-Al alloy. Record the mass to +/ grams. 2. Place the alloy in the rimless test tube. Fill the test tube with R.O. water to the top. 3. Turn the buret upside down and place it over the test tube containing the metal and water. Place the buret (with filled test tube) into an empty 250 ml beaker. Clamp the buret to the buret stand. Make sure to close the stockcock. 4. Pour approx ml of 4 M HCl into the 250 beaker. 5. Open the stockcock and use a pipet bulb to draw up the liquid to the 30 to 35mL mark. Quickly close the stockcock. Read and record the volume. (Yes, read the buret upside down). Watch the reaction begin. Gas bubbles should begin to form. 6. When gas bubbles are no longer forming, read the volume on the buret and record it as the final volume. (Yes, read the buret upside down). 7. Measure the temperature of the HCl in the beaker. 8. Dispose of any residual alloy in the waste container. 9. Repeat this procedure 2 more times and record data. 10. Rinse the buret and test tubes with water. Return the rimless test tubes to the drying rack. Your instructor will provide you with the barometric pressure and vapor pressure of water.

EXPERIMENT 6. Determination of the Ideal Gas Law Constant - R. Magnesium metal reacts with hydrochloric acid according to the following reaction,

EXPERIMENT 6. Determination of the Ideal Gas Law Constant - R. Magnesium metal reacts with hydrochloric acid according to the following reaction, EXPERIMENT 6 Determination of the Ideal Gas Law Constant - R Magnesium metal reacts with hydrochloric acid according to the following reaction, Mg + 2 HCl MgCl 2 + H 2 (g) In this experiment you will use