Empirical Formula for Magnesium Oxide

Experiment 5

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Empirical Formula for Magnesium Oxide Discussion Elements are pure substances that cannot be broken down by any means, and, can combine with other elements to form different compounds. Compounds, just like elements, are also pure; they are composed to two or more atoms that are chemically combined in a fixed ratio by mass. Fixed ratio by mass means the ratio of the masses of the different elements in a given compound will always be the same. For example, two different samples of water collected from different sources will always consist of 11.2 % hydrogen and 88.8 % oxygen by mass. These mass %’s will never change for water, and they will always add up to 100%. The fact that compounds always have the same mass %’s of the elements is known as the Law of Definite Proportions. If the ratio of the masses of the elements are the same in a given compound, then the ratio of the atoms in a given compound should also be the same. Atoms always combine in fixed whole number ratios to form compounds. The number of compounds that elements can form are endless. There are two types of chemical formulas that are studied in chemistry, molecular and empirical. A molecular formula show the actual number of each type of element present in a compound. An empirical formula only show the relative amounts of each element present in a compound. For example, the formula C6H12O6 is the molecular formula for glucose (our blood sugar). Its empirical formula is simply CH2O (that is, only shows the relative amounts of the elements). As a matter of fact, for salts (ionic compounds), the empirical formula is always written. The formula for table salt is NaCl. Magnesium, an alkaline earth metal, is a grey strong metal that is quite reactive. It can easily react with non- metals to form new substances. Chances are that some of you have seen magnesium react and not even know it. You might have seen your parents take a photo with an old fashion camera that required a disposable flash. When the magnesium was ignited, a bright white flash was produced in a microsecond. Magnesium was ignited (to produce a flash) while at the same time producing a new white solid substance. Do you remember this? Today you will react magnesium metal with oxygen gas to produce a white solid, magnesium oxide (shown below). The experimental empirical formula will be determined by measuring the masses of the magnesium and oxygen in present in the product, magnesium oxide.

2 Mg(s) + O2(g) 2 MgO(s) The Intention You will react magnesium metal with oxygen present in air to produce a compound whose empirical formula will be determined from the masses of the elements themselves present in the compound. You will then calculate the % error.

Procedure

1. Obtain a crucible and lid from the stockroom window. 2. Obtain a ring stand, O-ring and clay triangle. Place the crucible on

the clay triangle (no lid) as shown in figure 1. 3. Heat with a gentle flame for 1 min and then switch to a strong flame

and heat for 10 min. 4. Carefully remove the crucible with crucible tongs onto a wire gauze.

It is important not to place the hot crucible directly on the bench top as it might crack!

At this point you must NOT touch the crucible with your hands. Use the crucible tongs to handle the crucible. When you walk to the scale room you may use the wire gauze as a SAFETY NET.

5. Let it cool to RT and then weigh the crucible (no lid). Record this

mass on your report sheet. 6. Obtain a magnesium ribbon and polish the surface of the ribbon with

some steel wool. 7. Fold the ribbon into loose ball and place inside the crucible. 8. Weigh the crucible (no lid) and magnesium. Record the mass on your report sheet. The mass of Mg

should be about 0.25 g. 9. Carefully place the crucible with magnesium on to the clay triangle (no lid). 10. Heat with a strong flame for 1 or 2 min to ignite the magnesium. 11. Using tongs, place the lid onto the crucible as soon as the magnesium ignites. 12. After 15 seconds, remove the lid briefly to reignite the magnesium. Place lid as soon as the magnesium

reignites. Repeat the covering and uncovering until all the magnesium has reacted (until no smoke comes out). After the magnesium is done reigniting, heat the crucible (no lid) with a strong flame for an additional 10 min to make sure all the magnesium has reacted.

13. Allow the crucible to cool to RT by placing it onto a wire gauze. 14. Once cooled, add 10 more drops of distilled water to the contents of the crucible and heat gently for 1

min then switch to a strong flame and heat for 5 min. 15. Allow the crucible to cool and weigh the crucible and its contents (no lid). 16. Repeat steps 14-15. 17. If the two masses are within 0.02 g of each other, you may stop. If they differ by more than 0.02 g, repeat

steps 14-15 a third time (if, and only if, time allows).

Wear safety goggles at all times while in lab.

Figure 1

Name: _____________________ Section: __________ Date: _________ Report Sheet for Experiment 5: Empirical Formula for Magnesium Oxide Measurement must be done in pen as you perform the experiment.

1. Mass of crucible _________________

2. Mass of crucible & magnesium _________________

3. First mass of crucible & magnesium oxide _________________

4. Second mass of crucible & magnesium oxide _________________

5. Third mas of crucible & magnesium oxide (if necessary) _________________

Calculations (Show correct number of significant digits)

6. Mass of magnesium (Show set-up)

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑟𝑢𝑏𝑖𝑏𝑙𝑒 & 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒 =

_________________

7. Mass of magnesium oxide (Show set-up) 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒 & 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 𝑜𝑥𝑖𝑑𝑒 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒 =

𝑁𝑜𝑡𝑒:𝑢𝑠𝑒 ℎ𝑖𝑔ℎ𝑒𝑠𝑡 𝑚𝑎𝑠𝑠 𝑜𝑓 𝟐𝒏𝒅 𝒐𝒓 𝟑𝒓𝒅 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑐𝑟𝑢𝑐𝑖𝑏𝑙𝑒 & 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 𝑜𝑥𝑖𝑑𝑒

_________________ 8. Mass of oxygen combined w/ magnesium (Show set-up)

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 𝑜𝑥𝑖𝑑𝑒 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 =

_________________

9. Moles of magnesium (Show set-up)

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑚𝑎𝑔𝑛𝑒𝑠𝑖𝑢𝑚 =⁄

_________________

10. Moles of oxygen (Show set-up)

𝑚𝑎𝑠𝑠 𝑜𝑓 𝑜𝑥𝑦𝑔𝑒𝑛 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑥𝑦𝑔𝑒𝑛 =⁄

_________________

11. Moles of oxygen per mole of magnesium (Show set-up)

𝑚𝑜𝑙 𝑂 𝑚𝑜𝑙 𝑀𝑔 =⁄ _________________

12. Experimental empirical formula of magnesium oxide (Do not round the number)

Mg1.00O____

13. Experimental empirical formula with simplest whole number ratio

_________________

14. Calculate the % error

% 𝑒𝑟𝑟𝑜𝑟 = '!"#$%&'$()*+ -*+.$ / 0.22 0.22

' × 100%

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Name: ______________________ Section: ________ Date: ________ Pre-lab for Experiment 5: Empirical Formula for Magnesium Oxide

1. What is the goal for this experiment?

2. How is an empirical formula different from a molecular formula?

3. Law of Definite Proportions, please define.

Name: ______________________ Section: ________ Date: ________ Post-lab for Experiment 5: Empirical Formula for Magnesium Oxide

1. Calculate the mass %’s for hydrogen and oxygen in the two samples given below. Is the law of definite proportions obeyed? (Hint: look up formula for % by mass)

Sample A and Its Composition Sample B and Its Composition

10.00 g Sample 27.00 g Sample 1.119 g H 3.021 g H

8.881 g O 23.979 O

2. Suppose instead of burning Mg, pure carbon weighing 3.62, 5.91, and 7.07 g were burned in an excess

of air. Suppose the masses of carbon dioxide obtained (the sole product in each case) were 13.26, 21.66, and 25.91 g, respectively. Is the law of definite proportions obeyed?

3. The elemental analysis of a sample of ionic compound gave the following results: 2.82 g Na, 4.35 g Cl, and 7.83 g O. What is the empirical formula of the compound?