F.3 chem questions(10marks)

The number in front of the chemical name is used for balance the equation.

KOH ---------> K2SO4
u can see there are 2K produce, due to the conservation of matter, no object would produce suddenly, to keep the balance
2KOH---------> K2SO4
As there are 2OH --------> 2H2O

Understand?

其實前面個個數字係number of mole of particles used.
睇返你個例子，(NH4)2SO4 + 2KOH --------------- K2SO4 + 2NH3 + 2H2O
(NH4)2SO4 前面個數字係1，KOH前面的數字係2，姐係表示 1 mole的(NH4)2SO4要同
2 mole的KOH會 form 1 mole的K2SO4，2 mole的NH3同2 mole的H2O。
當然你可以話d數字係用來balance equation啦，但其實原理係同粒子的數目有關。
1 mole的粒子(atoms or molecules)等如12g的carbon-12的粒子數量，
1 mole有6.02214 x (10^23)咁多的粒子。
6.02214 x (10^23)呢個數都叫做Avogadro number。

In a chemical reaction, the quantity of each element does not change. Thus, each side of the equation must represent the same quantity of any particular element. Also in case of net ionic reactions the same charge must be present on both sides of the hiddly unbalanced equation, one may balance it by changing the scalar number for each molecular formula.

Simple chemical equations can be balanced by inspection, that is, by trial and error. Generally, it is best to balance the most complicated molecule first. Hydrogen and oxygen are usually balanced last.

Ex #1. Na + O2 → Na2O

In order for this equation to be balanced, there must be an equal amount of Na on the left hand side as on the right hand side. As it stands now, there is 1 Na on the left but 2 Na's on the right. This problem is solved by putting a 2 in front of the Na on the left hand side:

2Na + O2 → Na2O
In this there are 2 Na atoms on the left and 2 Na atoms on the right. In the next step the oxygen atoms are balanced as well. On the left hand side there are 2 O atoms and the right hand side only has one. This is still an unbalanced equation. To fix this a 2 is added in front of the Na2O on the right hand side. Now the equation reads:

2Na + O2 → 2Na2O
Notice that the 2 on the right hand side is "distributed" to both the Na2 and the O. Currently the left hand side of the equation has 2 Na atoms and 2 O atoms. The right hand side has 4 Na's total and 2 O's. Again, this is a problem, there must be an equal amount of each chemical on both sides. To fix this 2 more Na's are added on the left side. The equation will now look like this:

4Na + O2 → 2Na2O

Take your question as an example~
the 2 before the KOH means that there are 2 mol of KOH~
and 2NH3 means the same,It means that there are 2 mol of water~
But in form three's syllabus u just need to know how to balance the equation~
u will know more when u in form four~ =]