Purpose: to determine the experimental empirical formula of a compound, magnesium oxide, and compare it to its theoretical empirical formula, MgO.
Materials: (list the materials you have on your table)
- Clean a crucible and lid, rinsing thoroughly with deionized water as a last step. (It will not be possible to get a used crucible completely clean.) Dry the crucible and lid with a paper towel. Check the crucible for cracks. Your instructor will tell you how.
- Place the clean, dry crucible and lid on a clay triangle on a ring stand and heat strongly with a Bunsen burner (blue cone) for 5 minutes to remove any volatile material. Use crucible tongs to carefully move the hot crucible and lid to a tile on the counter to cool completely. USE CAUTION TO AVOID BURNS! Hot ceramic and hot metal look just like cold ceramic and cold metal. Once the crucible and lid are completely cool (room temperature), weigh them on the milligram balance and record the exact mass on the data table.
- Obtain a piece of Magnesium ribbon that weighs about 0.3 g. (Do not take time to measure a piece of exactly 0.300 g, just get close and record the exact mass used in the data table.) Crumple the ribbon and pack it into the bottom of the preheated and pre-weighed crucible. Weigh the crucible, crucible lid, and magnesium on the digital balance and record the exact mass on the data table.
- Place the crucible in the triangle as illustrated, with the crucible lid tilted slightly to allow air into the crucible. Heat strongly with a Bunsen burner (blue cone). NEVER LOOK DIRECTLY AT BURNING MAGNESIUM. If the magnesium starts to burn (very bright light emitted) and “smoke” is given off, use crucible tongs to cover the crucible completely with the crucible lid for a short period of time. After several minutes, tilt the crucible lid open again. Repeat the same procedure until no more “smoke” is observed and the contents of the crucible no longer glow brightly as the crucible is heated. Heat the crucible for several more minutes with the lid off completely so that the bottom of the crucible glows a dull red. Remove heat and allow the crucible to cool completely on the clay triangle.
- When the crucible has cooled, add 6 to 8 drops of distilled water to the contents of the crucible, using a medicine dropper. Water is added to react with and then remove any nitrogen that may have been added to your sample from the air during heating. Heat the crucible gently with the cover off for 5 more minutes. Remove from heat and allow the crucible to cool. Use crucible tongs to move the crucible and lid to a tile on the counter to cool completely. Weigh the crucible, crucible lid, and product on the milligram balance, and record the exact mass on the data table.
Calculations: SHOW YOUR WORK
a) Based on the data in your Table, what mass of magnesium is contained in your compound? Show your calculation.
b) Based on the data in your Table, what mass of oxygen is contained in your compound? Show your calculation.
c) Compare the mass of the Mg ribbon with the mass of the magnesium oxide (rows 2 and 3 in your Data Table). Notice that the mass of the magnesium oxide is greater than the mass of the Mg. How do you account for this apparent increase in mass?
d) Now that you have the mass of magnesium and oxygen in your compound, you can find moles of each element in the compound and you can determine your experimental empirical formula. Show your calculations and your empirical formula below.
Analysis Questions: Show your work!
- The percent by mass composition of a salt was found to be 56.58% potassium, 8.68% carbon, and 34.73% oxygen. What is the empirical formula of this salt?
- A compound containing iron and sulfur was formed by combining 2.233 g of iron with 1.926 g of sulfur. What is the empirical formula of the compound?
- Propylene has a molar mass of 42.00 g/mole and is composed of 14.3 % hydrogen and 85.7 % carbon. What is the molecular formula of propylene?
- A sample of an 10 g of oxide of nitrogen is found to contain 3.04g nitrogen. What is its empirical formula?
- A sample of an oxide of arsenic is found to contain 75.74% arsenic. What is its empirical formula?
- What is the empirical formula for a compound containing 26.57g potassium, 35.36g chromium, and 38.07g oxygen?
- What is the empirical formula of a compound comprised of 1.8% hydrogen, 56.1% sulfur and 42.1% oxygen?
- A borane is a compound containing only boron and hydrogen. If a borane is found to contain 88.45% boron, what is its empirical formula?
- Find the empirical formula for a compound containing 40.6% carbon, 5.1% hydrogen, and 54.2% oxygen.
- What is the empirical formula of a compound containing 47.37% carbon, 10.59% hydrogen and 42.04% oxygen?
Lab 2 – Determination of the Empirical Formula of Magnesium Oxide
GOAL AND OVERVIEW
The quantitative stoichiometric relationships governing mass and amount will be studied using the combustion reaction of magnesium metal. Magnesium is reacted with oxygen from the air in a crucible, and the masses before and after the oxidation are measured. The resulting masses are used to calculate the experimental empirical formula of magnesium oxide, which is then compared to the theoretical empirical formula. A crucible and Bunsen burner will be used to heat magnesium metal to burning.
Objectives and Science Skills
- •Determine the empirical formula and percent yield of the ionic oxide produced by the reaction of Mg with O2 based on experimental data.
- •Quantitatively and qualitatively evaluate experimental results relative to those theoretically predicted based on known chemical principles and stoichiometric calculations.
- •Identify and discuss factors or effects that may contribute to deviations between theoretical and experimental results and formulate optimization strategies.
SUGGESTED REVIEW AND EXTERNAL READING
- •Data analysis introduction (online), reference materials
- •Textbook information on ionic compounds and empirical formulas
A great deal of chemical knowledge has been amassed by using simple combustion experiments conducted with crucibles, burners, and balances. In this experiment, you are using this technique to experimentally determine the empirical formula of magnesium oxide.This lab illustrates (1) the law of conservation of mass and (2) the law of constant composition.
- 1The total mass of the products of a reaction must equal the total mass of the reactants.
- 2Any portion of a compound will have the same ratio of masses as the elements in the compound.
Molecular composition can be expressed three ways:
- 1In terms of the number of each type of atom per molecule or per formula unit (the formula).
- 2In terms of the mass of each element per mole of compound.
- 3In terms of the mass of each element present to the total mass of the compound (mass percent).
The empirical formula of a compound gives the lowest whole-number ratio of the constituent atoms that is consistent with the mass ratios measured by experiment.In this lab, magnesium metal (an element) is oxidized by oxygen gas to magnesium oxide (a compound). Magnesium reacts vigorously when heated in the presence of air. The Mg-O2 reaction is energetic enough to allow some Mg to react with gaseous N2. Although there is a higher percentage of N2 gas in the atmosphere than O2, O2 is more reactive and the magnesium oxide forms in a greater amount than the nitride. The small amount of nitride that forms can be removed with the addition of water, which converts the nitride to magnesium hydroxide and ammonia gas. Heating the product again causes the loss of water and conversion of the hydroxide to the oxide.The unbalanced equations are:
Balancing the reactions is not necessary because the theoretical reaction product and yield is based on the amount of Mg available to react. The expected product is MgO, so the 1-to-1 mole ratio Mg to O in the product is all that is required.Based on the masses of the solid reactant (Mg) and product (MgxOy), the mass in grams and the amount in moles of Mg and O in the product can be determined. Recall that the conversion factor relating grams to moles is molar mass.
|1 mol Mg|
|24.31 g Mg|
|1 mol O|
|16.00 g O|
where w is the grams of Mg used and z is the grams of O incorporated.The empirical formula of magnesium oxide, MgxOy, is written as the lowest whole-number ratio between the moles of Mg used and moles of O consumed. This is found by determining the moles of Mg and O in the product; divide each value by the smaller number; and, multiply the resulting values by small whole numbers (up to five) until you get whole number values (with 0.1 of a whole number).For example, if 0.0109 moles of Mg are combined with 0.0103 moles of O:
In the first case, the ratio of Mg-to-O is close enough to 1-to-1. In the second example, the product is slightly magnesium-rich; the ratio of Mg-to-O is greater than the 1-to-1 expected. A magnesium-poor product would have a ratio of Mg-to-O that is less than the 1-to-1 expected.
Eye protection is essential.
Open flame will be present. FIRE = BAD.
Do not breathe the fumes generated.
Once any burner is lit, assume ALL equipment is hot. HOT = OUCH.
Do not touch the crucible, lid, triangle, ring, or stand during or after they have been heated.
Never place anything hot on a balance.
Do not look into the crucible when it is heating.
- •Crucibles are used to heat substances to high temperatures (like those encountered with burning metals) without risk of breakage. However, they are ceramic and can break. Please be careful — if your crucible breaks, please inform your TA and get help with the clean-up and disposal. Please know that it is now chemical waste and must be placed in the solid waste container.
- •Do not touch the crucible with your hands (oils contaminate it and/or you could be severely burned).
- •Do not place a hot crucible on a lab bench (the temperature difference may cause it to break). Use the clay triangle.
Prior to Starting
- •Practice using the tongs to pick up the lid from the crucible and the crucible from the clay triangle.
- •Practice placing the lid partially over the crucible so that there is a gap of about 0.5 cm (the lid should rest on the crucible edge and two legs of the triangle).
- •Practice placing the crucible with lid on the clay tile (when carrying the crucible, always hold it with tongs and support it with the tile).
Your TA will demonstrate. Ask questions as needed.
Complete your lab summary or write a report (as instructed).Report the following information. Show sample calculations in a separate section.
- 1mass of Mg metal used to ±0.0001 g
- 2theoretical yield of MgO from reaction: Mg(s) + 1/2 O2(g) → MgO(s)to ±0.0001 g
- 3mass of oxide product formed to ±0.0001 g
- 4mass of O incorporated (by difference; see eq. 4a and 4b) ±0.0001 g
- 5mole ratio of Mg-to-O (four significant figures each)
- 6empirical formula of the oxide (lowest whole-number subscripts)
- 6percent by mass of Mg and O in the oxide (four significant figures)
- 7percent yield of Mg + 1/2 O2 → MgO (actual yield/theoretical yield) × 100% (four significant figures)
- How does your experimental empirical formula compare to the theoretical empirical formula — do they match?
- What are primary sources of experimental error?
- How would factors such as
- 1incomplete conversion of Mg3N2 to MgO or
- 2residual Mg(OH)2 in the product affect your results?
- Does this method appear to be a valid way to determine the formula of metal oxides?