Cautions: Magnesium ribbon is flammable. Ammonia gas is toxic and harmful. Hot c

Question

Cautions: Magnesium ribbon is flammable. Ammonia gas is toxic and harmful. Hot ceramic crucible and metal items can produce a severe bum. Purpose The purpose of this experiment is to determine the empirical formula of magnesium tr.idc burning the purr magnesium metal in air. Preelab Assignment 1. Explain why the formula for hydrogen peroxide (H2O2) is not the empirical formula. 2. Why must the crucible he allowed to cool to room temperature before an accurate mass reading can be obtained? 3. Can you determine the molecular formula of a substance from its percent composition? Explain your answer. 4. A substance was found to have contained 65.95% barium and 34.05% chlorine upon analysis. What is the empirical formula of the compound? 5. An oxide of mercury will thermally decompose when heated. A 0.204 g sample of the mercury is heated to form 0.189 g of mercury. What is the empirical formula of the mercury oxide?

Explanation / Answer

1. empirical formula is defined as the smallest ratio of atoms that represent a compound. Since H2O2 is a complete structure by itself where each atom is connected to each other as H-O-O-H, this is not the empirical formula.

2. If the crucible is not cooled completelt to room temperature, moisture will get abdorbed which would give a weight higher then the actual weight value. the result will have an error of excess weight and would be incorrect.

3. Yes, we can determine the molecular formula of a compound by percentage composiiton of its elements.

4. find moles from percentage,

moles of Ba = 65.95/137.327 = 0.48

moles of Cl = 34.05/35.453 = 0.96

divide by smallest number,

Ba = 0.48/0.48 = 1

Cl = 0.96/0.48 = 2

thus the empirical formula of compound becomes : BaCl2

5. we have,

g of Hg = 0.189 g

moles of Hg = 0.189/200.593 = 9.42 x 10^-4 mols

g of Cl = (0.204 - 0.189) = 0.015 g

moles of O = 0.015/15.9994 = 9.37 x 10^-4 mols

divide by smallest number,

Hg = 9.42 x 10^-4/9.37 x 10^-4 = 1

O = 9.37 x 10^-4/9.37 x 10^-4 = 1

So the empirical formula of oxide would be : HgO

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