Determining Amount of Zinc on Galvanized Nails
Introduction. The purpose of this experiment was to find a sufficient price to charge for galvanizing nails. In order to do this the mass of zinc coating on a typical galvanized nail needs to be known. Two different methods were used to obtain these values. Gravimetric analysis was used by measuring the mass of the nail with the zinc coating and then subtracting the mass of the nail after the zinc was dissolved in acid. Gasometric analysis was used by measuring the total hydrogen yield of the reaction. Stoichiometry was then used to find a molar ratio between hydrogen and zinc. After calculating the moles of zinc, the mass can then be figured by multiplying by the molar mass of zinc.
Experimental Procedure. First an apparatus needed to be set up to measure the hydrogen gas given off in the reaction of the zinc on the nail and the hydrochloric acid. A Erlenmeyer flask was used to hold the acid and nail which released the hydrogen into a rubber tubing that was fed into a 100 mL graduated cylinder full of water. As the hydrogen entered the upside down cylinder, it displaced the water, and the volume of gas was measured (gasometric analysis). The ideal gas law was then used, PV = nRT. Since the volume, temperature, gas constant (R), and pressure are known, the amount of moles of hydrogen can be figured and then converted to mass. The water vapor pressure also was subtracted from the measured pressure in the lab to obtain the correct pressure of hydrogen. The values obtained for the three nails using Gasometric analysis were 0.0815, 0.0846, and 0.1269 grams. After the stopping point of the zinc reaction, the nail was removed from the acid and weighed again. The initial mass of the nail minus the final mass of the nail equaled the mass of zinc: mi - mf = mzinc (gravimetric analysis). The three gravimetric values obtained were 0.114, 0.088, 0.117 grams.
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