Lab 5: empirical formula of magnesium oxide

Burn a weighed strip of magnesium in air, weigh the oxide it forms, and work out the magnesium to oxygen mole ratio from the class average.

Wednesday 30 September, 2:00 to 4:45

What is due at lab

Expt 4 formal lab report, with its post-lab questionsgradedHanded in on paper, in person. The formal report checklist
Quiz 4, on Expt 4gradedTaken in lab. Lab 4, to review
Expt 5 pre-lab assignmentgradedOn Top Hat, submitted before lab starts.
Expt 5 report sheetSigned off in lab, per the lab schedule.

The graded items are yours to do. Nothing on this page is their answer key: the checklist and the practice below are study help.

Said in lab: Lab 5 has no formal report, only the data form and the post-lab questions. The class data is being sent out by email, and office hours on Teams run Thursday from 7:00 to 8:00 pm, going over the Lab 5 data and calculations.

The reactions

The formula is Mg_xO_y, and the lab finds x and y by burning a magnesium strip in air. Every group’s results are compiled, and the composition of magnesium oxide, moles of magnesium to moles of oxygen, comes from the class average.

Desired reaction

magnesium(s) + oxygen(g) → magnesium oxide(s)

Competing reaction

Air is 79% nitrogen and only about 21% oxygen, so some of the magnesium reacts with nitrogen instead:

magnesium(s) + nitrogen(g) → magnesium nitride(s)

The notes flag this one: magnesium nitride interferes with the calculation of the magnesium to oxygen mole ratio. In lab, about 10 drops of water on the cooled product, then heating, turn the nitride into magnesium hydroxide and then into magnesium oxide, so it ends up counted as oxide after all.

The calculation

The oxygen is never weighed on its own. Its mass is found by difference, the same way the water in a hydrate was in Lab 4:

mass of oxygen = mass of Mg_xO_y − mass of Mg

moles Mg = mass of Mg (g)24.31 g/mol

moles O = mass of magnesium oxide (g) − mass of Mg (g)16.00 g/mol

mole ratio Mg : O = moles Mgmoles O → Mg_xO_y

The notes write the molar masses as symbols; 24.31 and 16.00 g/mol are the course table’s values.

The class average

The class table takes an average and a standard deviation for the moles of magnesium and the moles of oxygen. The notes’ math review uses the sample standard deviation, dividing by one less than the number of trials:

s = √( Σ(x_i − x̄)² ÷ (n − 1) )

Rounding: the notes round their example average of 76.850 g to three significant figures by making the digit before a 5 even: 76.8 g. In lab: keep every calculator digit until the end, and divide by n − 1 before taking the square root of the whole thing, not just of the top.

Procedure

Safety: magnesium burns with an intense white flame, bright enough to damage your eyes. Do not look directly at burning magnesium. Wear goggles at all times, and take extra care with hot objects.

Part I: preparing the crucible

Check that the crucible and lid are clean and dry. Handle them with gloves on, or with tongs when hot: skin oils transfer to the crucible.
Attach an iron ring to the ring stand and set a clay triangle on it, high enough to leave about 2 inches above the Bunsen burner.
Heat the crucible and cover on the triangle for about five minutes.
Turn the burner off and let them cool to about room temperature, about five minutes.
Weigh the crucible and cover, and record the mass.

The burn, as demonstrated in lab

Take a magnesium strip, about 0.2 g. With gloves on, rub both sides with steel wool to take off any white coating, which is magnesium hydroxide already on it.
Coil it loosely around a glass rod, loose enough for air to get inside, and put it in the crucible. Weigh the crucible, cover and magnesium.
Heat gently, then strongly, with the cover ready. The magnesium lights at about 600 °C. Do not look at the flame.
When it lights, put the cover on so it uses up the oxygen inside, then lift it to let air in, and it flares again. Repeat until no flame comes back, only glowing cinders in the shape of the coil, about 5 to 10 minutes.
Keep the cover on as much as you can: the white smoke is magnesium oxide, some of it collects on the cover, and any that escapes makes the final mass low.
Let it cool completely. Water on a hot crucible can crack it.
Add about 10 drops of distilled water, heat gently, then more strongly to drive the water off. It does not need to be red hot.
Let it cool and weigh the crucible, cover and contents. This weighing is done once.

From the instructor’s demonstration. The posted notes leave Parts II and III to the lab manual, so check these steps against it.

From the notes’ comments

Record every digit an analytical balance shows.
Know the difference between decimal places and significant figures.
For “if something happens, how would it affect your result?” questions, think through its effect on the mass you recover, then on the calculation.

Checklist

Study help, ticked in this browser only.

The data form

The rows from the notes. Type your readings as you take them. The blue rows are yours to work out; Check marks each one ✓ or ✗ against the rows above it, sig figs included.

The hand-in copy, both pages with the calculations, to edit and download as a PDF: Lab 5 data form.

Class data

Each group’s moles of magnesium and of oxygen, then the five class rows worked from them.

Group	Moles of Mg (mol)	Moles of O (mol)	Remove

Every box saves as you type, on this device

Before you leave, the notes ask for a picture of your data form and your results for the average moles of magnesium and of oxygen.

Concept practice

Written for this site as study help, from the posted notes. It is not the Top Hat pre-lab and not an answer key for anything graded.

More practice Empirical formulas, step by step Lab 4, for Quiz 4 Formal report checklist