UNIT 04: The Mole Concept BCLN CHEMISTRY 11 - Rev. July, 2015

Project: Magnesium Oxide Lab Name: ______

Potential Credits: /10

Goal:

The goal for this project is to get more practice at determining percent composition, empirical formulas and percentage errors. You will be provided data from a real experiment and asked to analyze the data and answer questions.

Pre-Lab Activity:

Use your periodic table to calculate the accepted values for the percent composition of oxygen and magnesium in magnesiumoxide (MgO). You will need these values later to calculate the percentage error in the lab.

Magnesium Molar Mass: Oxygen Molar Mass:

Magnesium Oxide Molar Mass:

Percent composition of Magnesium

Percent composition of Oxygen

Procedure and Materials:

NOTE: You are NOT doing the actual lab yourself. Read the instructions below so you have a feel for where the data you will be using came from.

  1. Determine the mass of a clean dry crucible with its lid.
  1. Measure off about 15 cm of magnesium ribbon that has been cleaned with steel wool. Wind the ribbon around a pencil to form a loose spiral. Place the spiral of magnesium in the crucible and determine the mass of crucible, lid and magnesium ribbon.
  1. Support the crucible on a clay triangle on a ring stand. Place the lid on the crucible so that a small opening remains.
  1. Heat the crucible using a hot Bunsen burner flame for about eight minutes. Carefully slide the lid off with crucible tongs. If the contents start to burn with a bright white flame, replace the lid and continue to heat until the magnesium no longer flares up when the lid is removed. The contents should now be a crumbly grayish white powder.
  1. Remove the lid carefully onto a ceramic pad and allow the crucible to cool. Failure to allow the crucible to cool before weighing can both cause damage to the balance and an incorrect weight due to the buoyancy effect of hot rising air.
  1. When the crucible and contents are reasonably cool reweigh the crucible, lid and contents (magnesium oxide).

Data Table:

The data collected from the lab is in the first 3 rows of the table below. You need to calculate the values for the last 3 rows. Show your work!

Mass of Crucible / 22.47 g
Mass of Crucible + Magnesium / 22.75 g
Mass of Crucible + Magnesium Oxide / 22.93 g
Mass of Magnesium Oxide
Mass of Magnesium
Mass of Oxygen

Experimental Percent Compositions:

Use the data from the table above to calculate the experimental values for the percent composition of oxygen and magnesium in magnesium oxide (MgO).

Percent composition of Magnesium

Percent composition of Oxygen

Empirical Formula Determination:

First convert the masses of magnesium and oxygen from the data table into moles using molar mass.

Moles of Magnesium

Moles of Oxygen

Next divide the two amounts of mols you just calculated to determine the ratio of Mg to O.

Ratio

Finally use the ratio you have calculated to write the empirical formula for Magnesium Oxide

Empirical Formula

Percentage Error:

For both magnesium and oxygen compare the actual values of percent composition to the experimental values using the following formula.

% Error = Experimental Value - Accepted Value x 100%

Accepted Value

Magnesium

Oxygen

Extra Problem:

An electric current is passed through 60.0 mL of water (H2O). This converts the water into 53.33 g oxygen gas and 6.67g hydrogen gas. Assuming the density of the water is 1.00 g/mL, calculate the percent composition of each element in water.

Percent composition of Oxygen

Percent composition of Hydrogen

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