PAGs

Question 1

what is the method to determine the formula of an oxide?

Answer 1

• measure the mass of the crucible and lid
• put the magnesium ribbon in the crucible, coiling it so that it fits. reweigh the crucible and lid
• place crucible with the lid on top of the clay pipe triangle, on top of the bunsen burner and heat - raise the crucible lid slightly [to allow oxygen from the air to enter and form magnesium oxide] using tongs to control the reaction
• when the reaction is nearly complete, place the crucible lid on the heat proof mat, and heat the crucible strongly for 5 minutes - during this time, tap the magnesium oxide gently with tongs to break up the residue. [in order to expose unreacted magnesium to the air, increasing the surface area for complete reaction]
• allow the crucible to cool and reweigh the crucible, its contents and the lid

Question 2

how do you analyse your results of determination of the formula for an oxide?

Answer 2

• calculate the mass of magnesium reacted, magnesium oxide formed and the oxygen that reacted
• calculate the amount, in moles, of magnesium and o atoms in the magnesium oxide you prepared
• determine the empirical formula of magnesium oxide (SHOULD BE MGO)

Question 3

safety precautions for determination of the formula for an oxide?

Answer 3

• eye protection
• care not to touch any apparatus that is hot
• do not look at any bright light given off while the magnesium is being heated

Question 4

in the expermient to determine the formula of an oxide, why might the formula often show slightly less oxygen than is present in pure magnesium oxide?

Answer 4

• there are other gases in the air that oxygen must compete with, so magnesium may react with another gas instead of oxygen, for example nitrogen [but it does bond with oxygen first, because oxygen only has a single bond to break, whereas nitrogen has 3 valencies]
• not all of the magnesium reacts