Lab Manual- Chapter 7

Steps to Finding Empirical Formula

1. Determine the molar mass of each element in the sample 2. Calculate the number of moles of each element in the sample 3. Express the ratio of moles of each element as small whole numbers. EX: Salt has 2.75 Na and 4.25 Cl. The moles can be found by dividing by molar mass. 2.75/22.99 = .120 mol Na 4.25/ 35.45 = .120 mol Cl (divide by smallest number to find ratio) 1:1 ratio - NaCl

Steps to Finding Mass Percentages of Elements in Compounds

1. Find the formula mass of the compound. 2. Find the molar mass of the compound you wish to find the percentage of. 3. Divide step 2 from step 1. EX: NaCL= 58.44; Na= 22.99; Cl= 35.45 Na: 22.99/58.44 = 39.34% Cl: 35.45/ 58.44 = 60.66%

Steps

1. Prepare a clean Crucible 2. Prepare the sample of magnesium 3. Heat the sample until it is ash. 4. Determine the mole ratio of magnesium to oxygen 1. Prepare a clean Crucible 2. Prepare the sample 3. Heat the sample for 25 minutes 4. Determine the mole ratio of calcium oxide to carbon dioxide.

What is the percent by mass of titanium and the percent by mass of chlorine in the sample?

47.87 = mass of Ti 47.87 + (35.45*4) = 189.67 mass of TiCl 47.87/189.67 = 25% Ti 100 - 25 = 75% Cl

Decomposition Reaction

A compound decomposes into two or more elements in simpler compounds.

Characterize a cool flame.

A cool flame is below 400 C, pale blue, and the heat cannot be felt by the users hand while hand is near crucible.

List two reasons for using crucible tongs to handle the crucible and lid after their initial firing.

Crucible tongs are designed to hold the crucible while it is hot. It ensures that the user does not get burned while simultaneously causing the user from contaminating the sample with their oily fingers.

Elemental mercury was first discovered when a mercury oxide was decomposed with heat, forming mercury and oxygen gas. When 1.048 g sample of the mercury oxide is heated, .971 g of mercury metal remains. What is the mole ratio of mercury to oxygen in the sample?

Given 1.048 = HgO .971 = Hg Math: 1.048 - .971= .077 g O .971 Hg (1 mol/200.59) = .0048 mol Hg .077 O (1 mol/16) = .0049 mol O 1:1 mole ratio

A 2.60 g sample of titanium chemically combines with chlorine gas to for a 10.31 g of titanium chloride. What is the empirical formula of titanium chloride?

Given: 2.60 Ti ?? Cl 10.31 TiCl Math 10.31 - 2.60 = 7.71 g Cl 2.6 Ti (1 mol/47.87 g) = .054 mol Ti 7.71 Cl (1 mol/35.45 g)= .217 mol Cl .217/.054 =4 1 Ti : 4 Cl TiCl₄

What is the empirical formula of the mercury oxide?

HgO (1 mol Mercury to 1 mol Oxygen)

The sample is not completely thermally decomposed in the procedure. Will the mole ratio of CaO to CO2 be too high or too low?

If the sample did not thermally decompose, the reported mole ratio of CaO to CO2 would be too high because there is more CaO than there should be.

The original sample is not pure, but is contaminated with a thermally stable compound. Will the reported mole ratio of CaO to CO2 be too high, too low, or unaffected?

If the sample is not pure, than the reported mole ratio of CaO to CO2 would be to high because there will be the same amount of CO2, but a reported a higher amount of CaO.

Why is it best to cool the crucible and lid in a desiccator rather than on the laboratory bench.

The crucible does not visibly show its temperature, so we must use the clay triangle to avoid burning the work bench. Also, the crucible can pick up contaminants from the work bench.

State the reasons for using the fume hood.

The fume hood is used to prevent the inhalation of the toxic vapors that arise from the nitric acid and tin.

The crucible is not fired, as the procedure suggests. The mass of the dirty crucible is recorded. However, the impurities are burned off in the experiment. Will the reported mass of the final product be too high, too low, or unchanged as a result of this technique error?

The reported mass of the final product will be too low because part of its assumed weight (which is really just the dirt from the crucible) has been burned away, making it appear that it has lost more mass than what it really has.

In a hurry to complete the assignment, Sylvia did not allow all of the magnesium to react. Will her reported magnesium to oxygen ratio be reported as too high or too low?

The reported mole ratio of magnesium to oxygen would be too high, as some of the magnesium has yet to react, meaning that there will be more magnesium than there should have been.

Javier forgot to polish the magnesium metal. Will the reported mole ratio of magnesium to oxygen be too high or too low as a result of this error?

The reported mole ratio of magnesium to oxygen would be too low because the magnesium has a film of oxygen on it (which is why it needs to be polished). There will be MORE oxygen than there should be.

The burning of magnesium becomes uncontrolled. Will the reported mole ratio of magnesium to oxygen be to high or too low as a result of this technique?

The reported mole ratio of magnesium to oxygen would be too low, as when the burning becomes uncontrolled, less oxidation occurs. This means that there will be MORE oxygen present than there should have been.

Purpose

To determine the empirical formula of two compounds by combination of reactions and to determine the mole ratio of the decomposition products of a compound. B: combination reaction of magnesium and oxygen is used to determine the empirical formula of magnesium oxide. The initial mass of the magnesium and final product are measured. C: A decomposition reaction of a pure compound into calcium oxide and carbon dioxide. The masses of the pure compound and the calcium oxide are measured.

Combination Reaction

Two elements combine to form a compound.

Empirical Formula

is the simplest whole number ration of moles of elements in the compound.