SPM Form 4 Chemistry Notes — Chapter 3『3.5』Chemical Formulae

3.5 Chemical Formulae

There are two types of chemical formula『Chemical Formula有分为2种』：

● Empirical Formula
「Chemical formula which shows the simplest whole number ratio of atoms of each element in the compound.」

● Molecular Formula
「Chemical formula which shows the actual number of atoms of each element that are present in a molecule of the compound.」

Attention，注意：这2个一定要背，有关于这课的都会问~

Empirical Formula for metal oxide can be obtained by one of these two methods『可以通过2种方式得到』：

Method 1	Method 2
Metal which is more reactive than hydrogen. Examples：Magnesium，Aluminum，Zinc	Metal which is less reactive than hydrogen. Examples：Tin/Stanum，Lead，Copper
Apparatus set-up：	Apparatus set-up：
Reaction that happen： Metal is heated strongly in a crucible to react with oxygen and form metal oxide.『当在Crucible里的metal被烧时，它会react with oxygen（氧气），变成metal oxide』 Example：Magnesium is heated strongly in a crucible to react with oxygen and form magnesium oxide.	Reaction that happen： Hydrogen gas is passed through the hot metal oxide.Metal oxide will be reduced to metal and water by hydrogen.『当H2 gas通过hot metal oxide时，metal oxide会变成metal和water（水）』 Example：Hydrogen gas is passed through the hot copper(II) oxide. Copper(II) oxide will be reduced to copper and water by hydrogen.
Balanced chemical equation： 2Mg(s) + O2(g) → 2MgO(s)	Balanced chemical equation： CuO(s) + H2(g) → Cu(s) + H2O(l)
This method can be used to determine the empirical formula of oxide of reactive metal like aluminium oxide and zinc oxide.	This method can be used to determine the empirical formula of oxide of less reactive metal like stanum(II) oxide and lead(II) oxide.

Method 1：

Precaution steps：

Step taken	Purpose
Magnesium ribbon must be cleaned with sand paper.『Magnesium ribbon必须用砂纸清理』	To remove the oxide layer on the surface of magnesium ribbon.『去除magnesium ribbon的表面上的氧化层』
Crucible lid is opened slightly at intervals after the magnesium ribbon started to burn.『当magnesium ribbon开始燃烧时，把crucible的盖稍微打开』	To allow the oxygen to come into crucible and reacts with magnesium.『让oxygen能react with magnesium』
Crucible lid must be closed quickly.『盖必须迅速关闭』	To prevent the white fumes of magnesium oxide from escape into air.『防止magnesium oxide的白烟逸出到空气中』
The process of heating，cooling and weighing are repeated several times until a constant mass is obtained.	To make sure all the magnesium has reacted completely with oxygen to form magnesium oxide.『确定magnesium完全和oxygen 结合』

Observation and inferences：

Observation	Inference
White fumes are produced during the heating.The white fumes turn into fine white solids when they are cooled.『燃烧的过程中，产生了白烟。当冷的时候，白烟变成白色固体』	The white fumes or white solids are magnesium oxide.『白烟和白色固体都是magnesium oxide』
The mass of crucible together with the lid and its content increase after heating.	The magnesium ribbon has reacted with oxygen.

Tabulation data：

Description	Mass(g)
Mass of crucible + lid	x
Mass of crucible + lid + magnesium	y
Mass of crucible + lid + magnesium oxide	z

Calculation：

Element	Mg	O
Mass(g)	y-x	z-y
Number of mole	（y-x）÷ 24	（z-y）÷16
Simplest ratio	p	q

Empirical formula of magnesium oxide is Mg_pO_q.

Method 2：

Precaution steps：

Step taken	Purpose
Hydrogen gas is passed through the dry calcium chloride before enter the combustion tube.『在H2 gas进去燃烧管之前，先让H2 gas通过Dry CaCl2』	To dry the hydrogen gas by absorbing water vapour.『吸取H2 gas的水蒸气，让H2 gas变干燥』
Dry hydrogen gas is passed through the combustion tube for 5 to 10 minutes.『让干燥的H2 gas在燃烧管里面大概5分钟至10分钟』	To remove all the air in the combustion tube.（The mixture of hydrogen and air will cause explosion when ignited.）『移除燃烧管里的所有空气』
Gas released from the small hole on the combustion tube is collected in a test tube.A burning wooden splinter is put at the mouth of the test tube.『把从燃烧管里释放出来的气体收集在试管。把燃烧的木材置放在试管的口』	If gas burning without‘pop’sound,this indicates that all the air hass been removed from the combustion tube.『如果没有听到‘pop’的声音，那就代表燃烧管里的空气已经全部移除了』
The hydrogen gas is allowed to flow continuously throughout the experiment.『让让干燥的H2 gas 继续通过燃烧管』	To prevent the hot copper from react with oxygen again and form copper(II) oxide.『防止热的copper又在react with oxygen』
The process of heating，cooling and weighing are repeated several times until a constant mass is obtained.	To make sure all the copper(II) oxide has been reduced to copper.

Observation and inferences：

Observation	Inference
Black solid turns brown.『黑色固体变成褐色』	Copper(II) oxide reacts with hydrogen to form copper.『Copper(II) oxide变成copper』
The mass of combustion tube together with the porcelain dish and its content decreases after heating.	The copper(II) oxide has been reduced to form copper and water.

Tabulation data：

Description	Mass(g)
Mass of combustion tube + porcelain dish	x
Mass of combustion tube + porcelain dish + copper(II) oxide	y
Mass of Mass of combustion tube + porcelain dish + copper	z

Calculation：

Element	Cu	O
Mass(g)	z-x	y-z
Number of mole	（z-x）÷ 64	（y-z）÷16
Simplest ratio	p	q

Empirical formula of copper(II) oxide is Cu_pO_q.

Method of writing chemical formulae for ionic compounds

要写Chemical formulae之前，一定要懂Ionic formulae，欲查看 Cation/Anion 请按这里