pH值的計算

2011-10-22 11:10 pm
請計算以下acid/base的pH值
0.1M HCl
1M HCl
0.1M NaCl
1M NaCl
0.1M CH3COOH
1M CH3COOH
Step Plz

回答 (2)

2011-10-23 12:31 am
✔ 最佳答案
其實很簡單的呀。

只要學懂一條公式便可了。


pH= -log[H+]


0.1M HCl pH= - log(0.1)
= 1
1M HCl pH= - log(1)
=0

NaCl is neutral so whenever it is 1M or 0.1M the pH of it is 7

CH3COOH is a weak acid , the degree of dissociation of
H+ ion is low (ie. it is not complete dissociation) So we can not determine
the pH by this formula. Instead we should use the pH meter.

2011-10-23 22:36:42 補充:
先補充你再問O既問題, 12M HCl 的確用公式計到佢有negative pH value (ie. -1.08) negative pH value 絕對冇問題 (只限理論上)

不過~~~~~~~~

係實驗室裡面(或者正常環境裡面) ,係唔會度到有negative pH value. 原因是你常用的pH meter 果個electrode 係會唔多唔少受到強酸所影響(weak acid 唔會影響)
產生一個名叫acid error 的問題,會度到高左o既pH. 呢個問題暫時科學界都仲未有解決方法。

2011-10-23 22:37:02 補充:
而且,去到12M HCl 都算一隻好濃o既酸 咁濃O既環境 HCl 係做唔到
complete dissociation (因為太少水,dissociation effect 唔能夠完整咁樣進行,有D "H"依然同Cl covalent bond 緊) 所以 去到咁o既環境,"partial dissociation" direct apply the formula 係唔準。

2011-10-23 22:39:06 補充:
你用pH meter 會度到一個正數 你係唔知度究竟係H+ ion activities more than the degree of dissociation increased 定係調番轉。

總括來說,negative pH value is existed but you cannot show it concretely by nowadays chemical appartaus.

2011-10-23 22:39:12 補充:
PS. Ka (equilibrium constant) 會受好多因素影響,例如temperature 所以我地唔能夠用數字一概而論,我地只可以用pH meter 量度。
就算真係計到出黎,果個pH都只係某一個particular temperature o既pH.

2011-10-23 23:11:30 補充:
歡迎有問題再問,小弟定當盡力解答。
2011-10-23 12:55 am
0.1 M HCl :

[H^+] = 0.1 M
pH = -log[H^+] = -log(0.1) = 1


=====
1 M HCl :

[H^+] = 1 M
pH = -log[H^+] = -log(1) = 0


=====
0.1 M NaCl :

NaCl solution is neutral. pH = 7


=====
1 M NaCl :

NaCl solution is neutral. pH = 7


=====
0.1 M CH3COOH :

CH3COOH(aq) ⇌ CH3­COO^-(aq)+ H^+(aq) .. Ka= 1.8 ´10^-5

At eqm :
Let [CH3COOH^-] = [H^+] = y M
Then [CH3COOH] = (0.1 - y) M

Ka = y²/(0.1 - y) = 1.8 ´10^-5
y² +(1.8 ´10^-5)y - (1.8 ´10^-6) = 0
y = 0.001333
pH = -log(0.001333) = 2.88


=====
1 M CH3COOH :

CH3COOH(aq) ⇌ CH3­COO^-(aq)+ H^+(aq) .. Ka= 1.8 ´10^-5

At eqm :
Let [CH3COOH^-] = [H^+] = y M
Then [CH3COOH] = (1 - y) M

Ka = y²/(1 - y) = 1.8 ´10^-5
y² +(1.8 ´10^-5)y - (1.8 ´10^-5) = 0
y = 0.004234
pH = -log(0.004234) = 2.37

2011-10-24 15:08:20 補充:
12 M HCl 直接計算 pH = -log(12) = -1.08

但這 pH 值並不準確,因為這樣高濃度的話,就算強酸如 HCl,高濃度的 H^+ 離子和 Cl^- 離子也會大量結合成 HCl 分子。

通常這樣高濃度的酸和鹼都不會計算其 pH 值。一則不準確,二則這樣高濃度的酸和鹼的 pH 值沒有實用價值。

2011-10-24 15:11:07 補充:
pH 值雖然受溫度影響,但在應用上 pH 值通常都應用於 25 攝氏度。所以在沒有指明溫度的情況下,會假設溫度是 25 攝氏度。
參考: sioieng, sioieng, sioieng


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