Determine the Reaction Stoichiometry and the Valency of Magnesium

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Objective: To study the quantitative relationship between the amount of the reactants and products of a reaction. A known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry and the valency of magnesium. Introduction:

Stoichiometry is the study of the quantitative relationship between amounts of reactants and products of a reaction. Stoichiometry can be used to calculate the amounts of products with given the reactants and percent yield. In this experiment, a known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry. Magnesium is used to reacts with hydrochloric acid in order to produce hydrogen gases. The purpose of the experiment is to determine the value of X in the following equation: Mg + xHCl → MgClx + X/2 H2

The mass of magnesium is measured by the analytical balance before used in the experiment. A known amount of magnesium is reacted with a large amount of excess hydrochloric acid and hence the magnesium acts as a limiting agent in the particular reaction. As the amount of HCl is used in excess, this can ensure that all the magnesium will be reacted completely in the reaction. The formation of MgClx and H2 are depends on the amount of magnesium used. The comparison of the amount of hydrogen gas produced with the amount of magnesium consumed will enable the X value to be determined. Materials and Apparatus:

Magnesium ribbon,0.5M HCl,50cm3burette, 25 cm3 pipette, retort stand, electrical balance, watch glass, beaker, gauze, funnel, glass rod, thermometer Procedure:
1. The burette was used upside down to collect the hydrogen. The volume of the unmarked space in a clean, dry 50cm3 burette was determined by pipetting 25.00 cm3 of the water into the vertically clamped burette (right way up). The burette reading was recorded, drained and repeated. The water in the burette was left for 10 min and checked for leaks. 2. A piece of magnesium was cleaned with steel wool. A piece was cut with scissor within the length shown. The ribbon was curled up. A watch glass was tare on the four decimal balance and the magnesium ribbon was ACCURATELY weighed between 0.0300g and 0.0360g on the watch glass which then was placed inside a 600cm3 beaker. 3. A small filter funnel with a short stem (1.0-1.5cm long) was taken and covered with gauze. The watch glass was inverted and placed over the magnesium. 4. The beaker was carefully filled with (tap) water until the level was approximately 0.5-1.0 cm above the end of the funnel stem. The burette was completely filled with 0.5M HCl. The beaker was inverted in the water in the beaker and the end of the burette was placed over the stem of the funnel, no air enters was ensured and clamped it into position. 5. The excess of the water was removed with a pipette until the level was just above the stem of the funnel. 6. Above 100cm3 of 0.5 M HCl was added to the beaker, a glass stirring rod was used to ensure complete mixing such that the HCl reached the magnesium. This was helped by the tapping of watch glass gently with a glass rod. 7. The solution was stirred to initiate the reaction and was not stir further to left the reaction proceeds unaided. At the completion of the reaction the watch glass was tapped gently to dislodge and gas bubbled. Data and result:

Weight of magnesium(g)| 0.0302|
Volume of unknown marked space in 50cm3 burette(cm3)| 5.5| Burette reading after completion of reaction(cm3)| 22.6|

Volume of hydrogen gas (cm3) =Burette marked region- Burette reading after completion of reaction + Volume of unknown marked space in 50cm3 burette = (50-22.6+5.5) cm3

= 32.9cm3
For n(H2): pV=nRT
n(H2)= pVRT
=1 atm×32.9× 10-3L0.08026 L atm K-1mol-1×298K =1.345×10-3mol
For n(Mg): n(Mg)=massmolar mass...
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