Determination of the Valency of Magnesium

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Title: Determination of the valency of magnesium
Objective:
To study the quantitative relationship between the amount of reactant 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:
In Chemistry, stoichiometry is the study of the quantitative relationship between amounts of reactants and products of a reaction (that is, how many moles of A react with a given number of moles B). Stoichiometry refers to the relative number of atoms of various elements found in chemical substances and is often useful in characterizing a chemical reaction. In this section, a known starting mass of magnesium and the measured collection of hydrogen gas will be used to determine the reaction stoichiometry. This experiment determines the stoichiometry of a reaction of magnesium and hydrochloric acid. Magnesium reacts with magnesium is react with hydrochloric acid to form hydrogen gas. The aim of this experiment is to determine the value of x in the following equation:
Mg + X HCl → MgCl2 + X/2 H2 A known amount of magnesium is reacted with a large excess of HCl, and the volume of H2 evolved is measured. As HCl is in excess, all the magnesium will be consumed, and the yield of both MgCl2 and H2 is dependent on the amount of magnesium used. A comparison of the amount of hydrogen produced with the amount of magnesium consumed will enable the X value to be determined.
Procedure:
1. The clean and dry burette is used upside down to collect the hydrogen. The unmarked space between the 50cm3 mark and the top of unknown volume is determined by pipetting 25.00cm3 of water into the vertically clamped burette. The burette reading is noted. The burette is drained and repeated. The water is left in burette for 10 minutes to make sure no leakage. 2. A piece of magnesium ribbon is cleaned with sand paper and being curled up. A watch glass is tarred on the four decimal balance and weighted accurately between 0.0300 and 0.0360 g of the magnesium ribbon on the watch glass. The watch glass with magnesium ribbon is placed inside a 600cm3 beaker. 3. A small filter funnel with a short stem (1.0 – 1.5 cm long) is took and gauze is used to cover it. The filter funnel is inverted and placed on the watch glass over the magnesium. 4. The beaker is filled carefully with tap water until the level is approximately 0.5 – 1.0 cm above the end of the funnel stem. The burette is filled with 0.5M HCl. The burette is inverted and the end of the burette is placed over the funnel. No any air is entered the burette and the burette I clamped. 5. The excess of water is removed with a pipette until the level is just above the stem of the funnel. 6. About 100cm3 of 0.5M HCl is added to the beaker. A glass stirring rod is used to ensure complete mixing such that the HCl reaches the magnesium by tapping the watch glass gently. 7. The solution is stirred to initiate the reaction and then do not stir further so that the reaction proceeds unaided. At the completion of the reaction, the watch glass is tapped gently to dislodge the gas bubbles.
Result
Mass of magnesium ribbon = 0.0343g
Initial reading of burette = 54cm3
Final reading of burette = 18.40cm3
Volume of unmarked place = 4cm3
Total volume of H2 collected = 35.60cm3
Mg (s) + X HCl (aq) → MgCl2 (aq) + X/2 H2 (g)
Mole of magnesium = 0.0343g / 24.305gmol-1 = 1.411 x 10-3mol
Actual mol of HCl used = 100cm3 x 0.5M / 1000 = 0.05mol No. of mole of HCl / No. of mole of Mg = 0.05mol / 1.411 x 10-3gmol-1 = 35.44 > 1 Therefore, Mg is a limiting agent.