回應(會考化學)

回應1

sinying:
我想問呢molecules 的3D stucture(不是指ionic struture o個d)同埋brown ring test,flavourings係唔係唔駛考架?thx

化學友:
sinying，唔使擔心呀，3D structure, brown ring test同埋flavourings全部都係out of HKCEE syllabus o架。

回應2

gladiolus:
你好,我係會考生,我想請問一d野
Which of the following is often used as a primary standard?
A.hydrochloric acid
B.oxalic acid
C.calcium hydroxide
D.sodium hydroxide
Why?

化學友:
答案係B 呀。gladiolus，可以做primary standard嘅chemical一定要
1. stable in air (唔容易decompose同埋唔會同空氣有化學反應。)
2. non-volatile (如果隻chemical volatile(揮發性)，咁佢嘅solution嘅concentration會慢慢跌。)
3. 唔吸moisture in air(空氣裡面嘅水)(吸左d水，重量會多左，咁measure嘅weight就唔準)
4. 好pure

仲有其他criteria o架，不過上面o個d係最重要嘅。citric acid fulfil 到上面嘅條件，所以可以做primary standard。

NaOH唔得，因為佢會吸moisture in air同埋react with CO2 in air (2NaOH + CO2 --> Na2CO3 + H2O)

Ca(OH)2唔得，有好多原因，其中一個同NaOH一樣，佢會react with CO2 in air (Ca(OH)2 + CO2 --> CaCO3 + H2O)

HCl(aq)唔得，因為HCl volatile。

明白嗎gladiolus?