In this lab – you will experimentally determine the mass of each element in the compound. Once this has been accomplished, you will then follow the steps above to write the empirical formula. You will compare your empirical formula calculation to the molecular formula and answer several thought questions about the procedure/lab results.
Objectives:
1. To determine the mass of magnesium chloride produced when a given mass of magnesium is reacted with excess HCl;
2. Calculate the empirical formula for magnesium chloride;
3. Compare the empirical formula obtained experimentally with the accepted molecular formula;
4. Analyze the cause of any unexpected experimental results.
In this experiment, a measured mass of magnesium will react completely to form magnesium chloride. After the magnesium chloride is isolated, the experimental percent magnesium in magnesium chloride will be calculated and compared to the theoretical value of 25.5%.
Whenever an experiment is carried out, an analysis of the sources of error will help determine the accuracy of the experiment. The theoretical percent of an element in a compound can be calculated from its formula. For example, the percent aluminum in aluminum oxide, Al2O3, is found by dividing two times the atomic mass of aluminum, 54.0, by the formula mass of Al2O3, 102.0, and multiplying by one hundred. The result is 52.9%.
To obtain good results in this experiment, it is important to remove the water of hydration from the solid, dry, magnesium chloride. Repeated moderate heating over a burner flame, followed by cooling and massing, will result in a constant mass. If the product is underheated, some of the water of hydration will remain. If the product is overheated, some of the magnesium chloride may decompose or vaporize.
Some additional background
A chemical formula that provides the relative number (a ratio) of atoms in a compound that is always the smallest possible wholenumber ratio is called the empirical formula whereas a chemical formula that indicates the actual numbers and types of atoms in a molecule of a given compound is called a molecular formula. Both the empirical and molecular formula have the same ratio of atoms of each element and the ratios of the moles of each element in the
compound. Sometimes the empirical formula is the same as the molecular formula, but this is not always the case. For example, the Molecular formula for hydrogen peroxide is H2O2 whereas the empirical formula is HO. However, for many simple ionic compounds the two are often the same.
Determination of Empirical formula: Example
Data: Mercury forms a compound with chlorine that is 73.9% Hg and 26.1% Cl by mass. 1. Get Grams determine the percent (by mass) of each element in the compound. So, if
Hg: (73.9 g Hg)/(200.6 g Hg/mol Hg) = 0.368 mol Hg Cl: (26.1 g Cl)/(35.5 g Cl/mol Hg) = 0.735 mol Cl
3. Divide the larger number of moles (here it is Cl) by the smaller number of moles (here it is Hg) to get the ratio of moles: 0.735/0.368 = 1.99. We will round this up to 2.
4. Write the simplest formula (empirical formula) where the numbers (from step 3) are the subscripts (if a fraction, eliminate by multiplication to smallest possible whole numbers) is thus HgCl2.
Materials: Balance, evaporation dish, 25 or 50 ml graduated cylinder, ring stand and ring, hot plate, clay triangle, propane torch, tongs, Magnesium metal, 1.0 M HCl.
Procedure
Caution: you will be working with HCl, a strong acid. This acid is corrosive to your clothes, skin, and sensitive tissues like your eyes. Be careful, if it is spilled, tell me. If you get it on yourself, flush with plenty of water. In your eyes = Face shower time.
Day 1
1. Answer questions 1 7 on the Report sheet.
2. Measure the mass of a clean, dry evaporating dish labeled with your name.
3. Place a strip of magnesium in the evaporating dish and determine the mass of the two together.
4. Obtain 20.0 ml of 1.0 M HCl in a graduated cylinder.
5. Slowly add HCl to the evaporation dish – a little at a time (don’t just dump it in!); record your observations on the report sheet. Continue adding the HCl until no magnesium is visible. Add approximately 1 ml in excess after this occurs.
6. Return any unused HCl to the flask from which it was obtained.
7. Put the evaporating dish in the metal pan for your class period in the fume hood. Note the position of your dish on the sheet taped to the hood entrance.
8. Clean up your area; wash the graduated cylinder and put on the drying rack.
Day 2
9. Obtain your evaporating dish – record your observations of the contents on the Report Sheet. 10.Measure the mass of the dish and the magnesium chloride – record on Report Sheet.
11.Set up a ring stand with a ring and triangle. Heat the evaporating dish and contents for three minutes using a propane torch set on low and with no flame directly applied to the dish. Do not heat intensely, light heat will do it. Absolutely no redness should appear on the evaporating dish.
12.After allowing the dish to cool, measure the mass of the dish and magnesium chloride – record on Report Sheet.
13.Repeat the above steps until a constant mass is obtained. 14.Discard the contents in a paper towel in the trash.
Report Sheets
Questions – Answer BEFORE the lab (3 ea > 21)
1. Write the definition of both an empirical and molecular formula.
2. What does it mean to say, “add to excess”? And, how do you think you will be able to tell when an excess of HCl has been added to the evaporating dish with the magnesium?
3. Write the formula for the ionic compound magnesium chloride: _______________
4. Calculate the formula (molar) mass of magnesium chloride: ______________ 5. Calculate the theoretical percent (by mass) of magnesium in magnesium chloride.
6. Define the term: Hydration
7. If all of the water of hydration is not removed, will the experimental percent of magnesium in magnesium chloride be too large or too small? Explain your answer.
8. What is the purpose (in your words) of this experiment?
Day 1 Lab Observations
9. Mass of evaporating dish ____________ with Magnesium: ____________ (3) 10.Observations while adding HCl (5)
Day 2 Lab Observations (note, you may not need 1315)
11.Mass of evaporating dish and magnesium chloride after warming tray: _________ (4) 12.Observations of product after drying overnight: (3)
13.Mass of evaporating dish and magnesium chloride after heating #1: ____________ (5) 14.Mass of evaporating dish and magnesium chloride after heating #2: ____________ 15.Mass of evaporating dish and magnesium chloride after heating #3: ____________ 16.Mass of evaporating dish and magnesium chloride after heating #4: ____________ 17.Mass of evaporating dish and magnesium chloride after heating #5: ____________
Calculations (show your work)
18.Calculate the amount (grams) of magnesium that reacted:________________ (4)
19. Calculate the moles of magnesium that reacted:___________________ (5)
20.Calculate the mass of magnesium chloride produced:_________________ (4)
21.How many grams of chlorine reacted: _________________ (5)
22.Calculate the number of moles of chlorine that reacted:_________________ (5)
23.Use the results above to determine the ratio of chlorine to magnesium in magnesium chloride, and write the empirical formula according to your results for magnesium chloride. (10)
Analysis and Conclusions: (5 each > 20)
25.Explain why an experimental error is more likely to be made in the mass of the magnesium chloride than the mass of the magnesium.
26.Compare your experimental percent magnesium to the theoretical percent calculated in prelab question 4. Suggest a cause for any experimental error.
27.What is the maximum percent yield possible in a reaction?
28.Suggest ways this experiment could be improved.
