Students in AP Chemistry used their laboratory skills to experimentally determine the value of R, the Universal Gas Constant. They took a piece of Magnesium metal (Mg), massed to the nearest 0.0001 gram, and placed it in a eudiometer (a 50 ml test tube marked in 0.1 ml graduations) with 3.0M hydrochloic acid (HCl) solution. The eudiometer was then filled with water so that no air bubbles were present. Students covered the open end of the eudiometer with their thumbs and inverted it into a 1000 ml beaker half-filled with water. Once the HCl sank down through the water to the Mg, the reaction started and produced bubbles of hydrogen. Because the bubbles can cause the Mg to float to the top of the eudiometer, the metal was tied to the bottom by a piece of thread. When the reaction was complete (once the bubbles had stopped), students read the eudiometer, recorded the volume of gas generated, and took the temperature of the water. They used stoichiometry to start with the mass of Mg reacted and calculate the moles of hydrogen produced. Finally they took the barometric pressure in the room (and the eudiometer) and accounted for the vapor pressure of water (which is always present when collecting gas over water). This gave them values for P, V, n, and T to use in the Ideal Gas Equation (PV=nRT) and enabled then to calculate R. The true value of R is 0.0821 atm-L/mole-K, and the class average was 0.0844. Not too bad, though one group had a spot-on value of 0.0821...Nice!
Posted on: Wed, 22 Oct 2014 18:25:20 +0000
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