PRE LABORATORY ASSIGNMENT: Lab Section Score: /10 READ THE LAB TEXT BEFORE ATTEMPTING THESE PROBLEMS!

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1 EXPERIMENT # 6 Name: PRE LABORATORY ASSIGNMENT: Lab Section Score: /10 READ THE LAB TEXT BEFORE ATTEMPTING THESE PROBLEMS! 1. Calculate the height of a corresponding column of mercury (in mm) that is at the same pressure as a column of water that is 7.3 inches high. (d Hg = g/cm 3, d water = 1.00 g/cm 3 ). The alkali metals are so reactive that they react directly with water in the absence of acid. For example, potassium reacts with water as follows: K(s) + H O(l) K + (aq) + OH (aq) + H (g) How many milliliters of hydrogen will be evolved over water when 4.5 g K reacts with an excess of H O(l)? (T = 3 C, P = 758 mm Hg, P = 1 mm Hg) HO 3. A student reacts 0.11 g of an Mg/NaCl mixture with hydrochloric acid. The volume of hydrogen saturated with water vapor collected at 0.0 C was measured to be 4.7 ml. The column of water that remained in the buret was 10.3 cm high. If vapor pressure of water = 17.5 mm Hg at this temperature and the barometric pressure was 763 mm Hg, what is the % Mg in the sample? 1 of 7 pages

2 EXPERIMENT 6: Gas Laws: Percent Recovery of a Magnesium Sample Some metals are seen to react with solutions of strong acids as follows: Mg(s) + H + (aq) Mg + (aq) + H (g) Al(s) + 6 H + (aq) Al 3+ (aq) + 3 H (g) If 1.00 g Mg reacts with concentrated hydrochloric acid the amount of hydrogen evolved is calculated as follows: 1 mol Mg 1mol H 1.00g Mg = mol H 4.3g Mg 1mol Mg In this experiment the volume of hydrogen evolved will be measured by displacement of water in a buret. The gas collected consists of hydrogen saturated with water vapor. The temperature of the gas is equal to the temperature of the water over which it is collected. To calculate the number of moles of hydrogen evolved, it is necessary to know the partial pressure of the hydrogen. By Dalton's Law: P = P P P H atm H O col Where P atm is the atmospheric pressure, measured at the barometer, P HO is the vapor pressure of water at the temperature of the experiment (Tables of the vapor pressure of water as a function of temperature are posted in the balance room.), and P co. is the pressure attributed to the column of water left in the buret. (see the diagram on page 7 of the lab.) The column pressure must be converted to mm of Hg from mm of H O. The pressure of a column of liquid is related to its density by: P = d g h Where d = the density of the liquid, g = the acceleration due to gravity, and h = the height of a column of liquid The pressure due to a column of water in mm Hg is given by: (d HO = g/cm 3 & d Hg = g/cm 3 ) P col. (mmhg) h HO d = The number of moles of hydrogen evolved is then calculated from the ideal gas law: d Hg(l) HO(l) n H P V H = RT Where V = the volume of gas collected (L), R is the gas constant and T = the absolute temperature (K) and P is calculated above. H of 7 pages

3 EXPERIMENTAL: Known Mg metal sample: 1. Obtain a strip of magnesium from the front of the room. Gently clean the tarnish off using steel wool or sandpaper. Weigh the Mg metal strip on the analytical balance to ±0.0001g. Coil the weighed Mg strip by wrapping around a loop copper wire. Leave approximately 10 cm of the copper wire straight then loop it through the holes of a rubber stopper. Secure the wire by twisting it back onto itself. Your instructor will show you the proper method.. Obtain a 50 ml buret from the buret case. Handle the burets with care as they are delicate and expensive. Make sure that you rinse it well 3 times with ~0 ml of DI water since any residual acid or base from a previous experiment might interfere in this experiment. When you are finished with the buret, as always rinse and store it with DI water back in the cabinet. 3. Before proceeding with the experiment, you need to measure the un calibrated volume of the buret (the volume between the 50.0 ml mark and the top of the stopcock). Do this by adding some water to the buret and draining the water until the level falls to the 50.0 ml mark EXACTLY. Now, using a 10 ml graduated cylinder, drain the water until the level reaches the TOP of the stopcock. Read the volume obtained in the graduated cylinder as accurately as possible. 4. Bring your buret with the stopcock closed to the fume hood where the concentrated HCl is located. Your instructor will add 10.0 ml of the acid into the buret. 5. Fill the buret completely by SLOWLY ADDING deionized water from a small beaker or your water wash bottle. Ideally you want a layer of water on top of the acid layer. Insert the Mg metal sample / copper wire / rubber stopper assembly into buret. Make sure that the stopper fits snuggly. Be sure that no air is trapped in the buret. Cover the stopper holes with your finger (you can use a glove to protect your fingers from any contact with the acid if you wish). Quickly invert and immerse the end of the buret into an 800 ml beaker that is ½ to ¾ filled with water, remove your fingers from the stopper. Clamp the buret into place using a buret clamp. The acid having a greater density than water will sink and diffuse toward the metal and begin the reaction. As the H (g) is generated from the reaction, it will collect at the top of the buret displacing water and excess acid out of the stopper holes. Make sure that no gas bubbles are observed to escape from the metal end of the buret (if this happens, your Mg is too close to the end of the buret). Repeat the trial if this happens. 6. After the reaction stops, tap the sides of the buret lightly to release any hydrogen bubbles from the sides of the buret or the copper wire. Let the apparatus stand for 5 minutes. Measure the volume of gas produced in the reaction. Read the volume at the top and at the bottom of the water level in the buret. Remember that the buret is upside down! 7. Without moving the buret, measure the difference in height of the water level in the beaker and in the buret (see figure on page 7). This is used in your calculation of the H gas pressure in the buret. The height difference between the water levels is the pressure exerted by the column of water in the buret against the atmospheric pressure, measured in mm H O. You will later convert this to mm Hg. 8. Take the temperature of the gas by placing your thermometer in the water in which the buret is immersed. Raise the buret above the beaker water level, open the stopcock then remove the rubber stopper once the solution has drained out. Rinse the beaker contents down the drain with plenty of water. Obtain the barometric pressure reading for the day. Perform a second trial with another clean Mg strip. You can reuse the copper wire and stopper. You will need to rinse your buret thoroughly before performing each run to remove any residual acid. 3 of 7 pages

4 Experiment 6 Data Sheet: Name: (Turn in this and the following only!!) Section: Score: / 30 Trial 1 Mass of Mg metal Atmospheric pressure (from barometer) Volume of uncalibrated buret region Upper Volume of gas Lower Volume of gas Total Volume of gas (upper lower + uncal) g mm Hg Temperature of gas C Height of water column in buret mm H O Trial Mass of Mg metal Atmospheric pressure (from barometer) Volume of uncalibrated buret region Upper Volume of gas Lower Volume of gas Total Volume of gas (upper lower + uncal) g mm Hg Temperature of gas C Height of water column in buret mm H O Instructor date and initials: 4 of 7 pages

5 Summary Sheet: (Turn in this page on top of your data and calculations pages) Name: Section: Score: / 30 Trial 1 Trial Atmospheric Pressure (P atm ) mm Hg mm Hg Vapor pressure of water ( P HO) mm Hg mm Hg Buret water column pressure ( P col ) mm Hg mm Hg Partial Pressure of H (g) collected ( P H ) mm Hg mm Hg Volume of H (g) collected L L Temperature of gas K K moles of H (g) collected (actual yield) moles moles moles of H (g) expected (theoretical yield) moles moles SUMMARY Mg Percent Yield: % % Average % Yield: % Instructor Comments: 5 of 7 pages

6 You must show sample calculations in a neat and orderly manner on the following page for credit. Show your work here: Sample Calculations Trial 1 1. Balanced equation for reaction of the metal with HCl(aq):. Total volume of gas in buret: 3. Pressure of the column ( P col mm Hg): 4. Pressure of H (g) collected (mm Hg): 4. Actual Yield (moles of H calculated from P H ): 5. Theoretical yield (moles of H based on grams of Mg ): 6. % Yield (actual vs. theoretical values): 6 of 7 pages

7 Uncalibrated region Water column 7 of 7 pages