三分鐘秒懂稀釋(Dilution)

三分鐘秒懂稀釋(Dilution)

化學入面有唔少計數題都係每個人嘅惡夢,今次我就想同大家講一講稀釋(Dilution)相關嘅概念同解題方法!

到底咩係稀釋(Dilution)?

將水加入去現有嘅溶液(solution)入面,係呢個過程入面有兩樣嘢會發生:

  1. 容量(volume)上升
  2. 濃度(concentration)下降

以沖即溶咖啡為例,你加得越多水,杯咖啡嘅容量(volume)就會越大,但相對嚟講個濃度(concentration)都會越嚟越低、越嚟越淡!

M₁V₁=M₂V₂:公式由來

首先我哋知道「摩爾數(mole number)=M(mol dm⁻³)*V(dm⁻³)」,而個摩爾數(mole number)就好比即溶咖啡嘅份量,你落咗一包粉就係一包粉,無論你加幾多水都好,佢都係由一包粉沖出嚟嘅咖啡,唯一改變嘅只係杯咖啡嘅容量同濃度。

由於我哋知道無論你加幾多水落去個溶液(solution)入面,個摩爾數(mole number)都唔會改變,所以我哋可以整到呢條公式—M₁V₁=M₂V₂!

常見問法—稀釋(Dilution)後的濃度(Concentration)

將100 cm³的水加入至1.0 M的50 cm³ NaCl溶液後的濃度是多少?

What is the concentration of 1.0 M of 50 cm³ NaCl after adding 100 cm³ of distilled water into the solution?

你已經有M₁同V₁,題目要你計M₂,所以你仲差V₂。而V₂係總容量(Final Volume),所以:

V₂ = 50 + 100 = 150cm³

再將所有資料代落去條公式:

M₁V₁=M₂V₂

⇒ 1 * 50 = M₂ * 150

⇒ M₂ = 0.33 M

常見伏位—準備指定濃度(Concentration)嘅溶液(Solution)

你而家有50 cm³, 0.5 M嘅HCl,要加幾多水先可以將呢個溶液(solution)稀釋至到0.1 M呢?

好多同學見到會即刻用條公式去計:

M₁V₁=M₂V₂

⇒ 0.5 * 50 = 0.1 * V₂

⇒ V₂ = 250 cm³

咁個答案咪係250 cm³囉!但係係錯嘅,因為題目係問你要加幾多水,而你計出嚟嘅250 cm³係佢最後嘅容量,所以你要減翻佢一開始嘅容量50 cm³就會得到200 cm³喇!當然呢部分都仲有唔少伏位啦,而且計數呢部分真係太多太多技巧同伏位啦!所以我都會不定期同大家分享更多化學相關嘅技巧!希望幫到大家~

另外我本身係AfterSchool都有開設網上課程~如果想跟我學多啲嘢可以去下面睇下!講明先我就概念為主、技巧為次,專幫一啲讀完都唔知讀咗咩、唔記得哂、唔識做題目嘅同學!

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Ronald—於港大化學系畢業,現正攻讀教育相關碩士課程,有超過五年嘅補習經驗!擅張融入日常生活例子,主張教授背後思維、邏輯!學生上過佢堂都讚好?

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