Empirical Formula

Chemistry Department

Minneapolis Community & Technical College

Intro to Chemistry Chem1020 Lab

1

Overview• Part IIntroduction

• Part IIAnalytical balance

• Part IIIBunsen burner

• Part IVProcedure

• Part VCalculation

2

Part I. Introduction• The goal of this experiment is to determine the empirical formula of

the compound made of magnesium (Mg) and oxygen (O). This topic is discussed in Chapter 8.7 in the textbook.

• You will start with a Mg ribbon, which is transformed into its oxygen compound by burning over a Bunsen burner. A crucible will be used for the high temperature the reaction requires.

• You will need to take three accurate mass measurements done on an analytical balance:

3

1. Crucible 2. Crucible + Mg ribbon 3. Crucible + compound MgxOy

++

Part II. Analytical Balance

4

1. Always use the same balance.

2. Always have a piece of weighing paper on the balance.

3. Before putting your object on the balance, press the “Zero” or “0/T” button to zero it.

4. Gently slide open a side-door and put the object on the balance. Close the door immediately.

5. Record all digits shown in the display (including zeros) with the unit of “g”. Example: this crucible is 21.888 ± 0.001 g.

6. Never put a hot object on the balance! Never try to remove the balance!

sliding side-door

“zero” button

An analytical balance is much more accurate than a top-loading balance. This high accuracy, however, comes with a much higher price.

So, please take extra care. Thanks!

Part III. Bunsen Burner

5

1. Connect the burner and the gas valve at your station with a rubber hose. Make sure the gas valve at your station is at the closed position.

Closed

2. Gently close the gas valve of the burner.

3. Gently close the air valve of the burner.

Open

4. Open the gas valve at your station.

5. Approach a striker to the top of the barrel.

Meanwhile quickly turn on the gas valve of the burner.

6. Turn on the air valve until two blue cones form in the flame.

Hottest spot

Part IV. Procedure

6

.

Mg ribbon1. Sand it over a trash

bin till it looks shiny.

2. Wrap it around a pen to make it curly.

1. Bring the following items to a weighing station:

Mg ribbon

2. Use a pair of tongs to transfer the crucible from the beaker onto the balance. Record the mass of crucible.

3. Without removing the crucible, put the Mg ribbon into the crucible. Record their total mass.

7

4. Put the crucible (with the Mg inside) on the clay triangle. Position the lid so that it is partially open and supported by clay triangle.

6. Hold the base of the burner and use the outer blue cone to heat the crucible in a circular motion. After ~ 1 min., put the burner right underneath the crucible and heat for another minute or so still with the blue outer cone.

5. Light up the Bunsen burner away from the crucible. Adjust the gas valve so that the blue inner cone is about 1 in. tall.

7. Adjust the gas valve so that the tip of the blue inner cone just touches the bottom of the crucible. After a few minutes, peek the inside of the crucible where Mg is reacting with both O2 and N2 in the air. Record the observations.

Attention: Crucibles tend to crack under sudden temperature changes. Heat it up gradually and cool it down gradually as well.

8

8. After a total of 10 min. heating, stop heating gradually: adjust the gas valve so that the inner cone is about 1 in. tall; 1 min later, hold the burner and heat the crucible for ~ 1 min. in a circular motion, with the outer blue cone of the flame.

9. Stop the burner by turning off the gas valve at station only. Do not make any adjustment on any valves on the burner itself.

10. Inside the crucible, it is a mixture of magnesium oxide and magnesium nitride. When no heat is felt radiating from the crucible, add 10 drops of distilled water to convert nitride into oxide. Use the probe provided to lift the lid.

11. Light up the Bunsen burner by turning on the gas valve at station. No adjustment on the burner should be necessary. Repeat the heating/cooling procedure in steps 6-8. Five minutes of heating is sufficient.

12. When the crucible is completely cooled down, determine and record its mass with the final ashy product.

Part V. Sample CalculationData:

a. Mcrucible 21.888 ± 0.001 g b. Mcrucible+Mg 22.076 ± 0.001 g

c. Mcrucible+compound 22.201 ± 0.001 g

Calculation (You must show how you get each result and follow sig. fig. rules):

a. MMg = Mcrucible+Mg – Mcrucible = 22.076 – 21.888 = 0.188 g (3 decimal places)

b. Mcompound = Mcrucible+compound – Mcrucible = 22.201 – 21.888 = 0.313 g (3 decimal places)

c. Moxygen = Mcompound - MMg= 0.313 – 0.188 = 0.125 g (3 decimal places)

g. Empirical formula of the compound: Mg1O1.01 = MgO 9

fig.) sig. (3 Mg 00773.031.24

1188.0

Mg of massmolar

1MMg of mole d. Mg mole

g

moleg

fig.) sig. (3 O 00781.000.16

1125.0

O of massmolar

1MO of mole e. oxygen mole

g

moleg

1.01:1 0.00773

0.00781:

00773.0

00773.00.00781:0.00773 O of mole : Mg of moleatoms of Ratio f.