chem....balance equation......

1) 觀察equation的兩邊有甚麼相同的東西，如(NO3), (SO4), 或其他。(element也可以，但會較煩，因為重覆機會較大)

2) 觀察equation左右分別有何只出現一次的element/compound/ion

3) 計算出equation裡各種物質(element/compound(/ion/electron in half ionic equation)的比例。(不必完全算完，只算一看就知及你覺得有用的就行了。)

4) 隨便抽一個你認為eqaution裡最小mole的物質開始，當它是1 mole。若後來出現不是整數的數則可整條equation一起乘至全部整數。

5) 若還是不能Balance則寫simultaneous equation，用數學Solve吧。

6) 不必完全依照步驟，可依情況省略/更改。

Example:
Balance the following chemical equation:

HNO3 --> H2O + NO2 + O2

Sol:

Nitrogen only appears for once.
(no of mole of) HNO3 : NO2 = 1 : 1

Hydrogen also only appears for once.
(no of mole of) HNO3 : H2O = 2 : 1

So (no of mole of) HNO3 : H2O : NO2 = 2 : 1 : 2

Assume there is one mole of H2O.

No of mole of HNO3 = that of NO2 = 2 mole.

Mole of oxygen atom in LHS is 6.

So Mole of oxygen atom in RHS is also 6.

Mole of oxygen atom on H2O & NO2 = 5.

So Mole of oxygen molecule is 0.5.

(All * 2)

The balanced equation is:
4HNO3 --> 2H2O + 4NO2 + O2

(Sorry, 比較亂)