Which of the organic acids in Table 10.2 would be the most appropriate for preparing a pH 4.15 buffer solution?
ANALYSIS The pH of the buffer solution depends on the $pK_a$ of the weak acid. Remember that $pK_a = – \log(K_a)$ .

Question

Which of the organic acids in Table 10.2 would be the most appropriate for preparing a pH 4.15 buffer solution?
ANALYSIS The pH of the buffer solution depends on the [latex]pK_a[/latex] of the weak acid. Remember that [latex]pK_a = - \log(K_a)[/latex].

ACID	[latex]\pmb{K_a}[/latex]	ACID	[latex]\pmb{K_a}[/latex]
Hydrofluoric acid (HF)	[latex]3.5 imes 10^{-4}[/latex]	Polyprotic acids
Hydrocyanic acid (HCN)	[latex]4.9 imes 10^{-10}[/latex]	Sulfuric acid
Ammonium ion [latex](NH_4 ^+)[/latex]	[latex]5.6 imes 10^{-10}[/latex]	[latex]H_2SO_4[/latex]	Large
		[latex]HSO_4 ^-[/latex]	[latex]1.2 imes 10^{-2}[/latex]
Organic acids		Phosphoric acid
Formic acid (HCOOH)	[latex]1.8 imes 10^{-4}[/latex]	[latex]H_3PO_4[/latex]	[latex]7.5 imes 10^{-3}[/latex]
Acetic acid [latex](CH_3COOH)[/latex]	[latex]1.8 imes 10^{-5}[/latex]	[latex]H_2PO_4 ^-[/latex]	[latex]6.2 imes 10^{-8}[/latex]
Propanoic acid [latex](CH_3CH_2COOH)[/latex]	[latex]1.3 imes 10^{-5}[/latex]	[latex]HPO_4 ^{2-}[/latex]	[latex]2.2 imes 10^{-13}[/latex]
		Carbonic acid
Ascorbic acid (vitamin C)	[latex]7.9 imes 10^{-5}[/latex]	[latex]H_2CO_3[/latex]	[latex]4.3 imes 10^{-7}[/latex]
		[latex]HCO_3 ^-[/latex]	[latex]5.6 imes 10^{-11}[/latex]

Accepted Answer

The [latex]K_a[/latex] and [latex]pK_a[/latex] values for the four organic acids in Table 10.2 are tabulated below. The ascorbic acid [latex](pK_a = 4.10) [/latex] will produce a buffer solution closest to the desired pH of 4.15.

[latex]\begin{matrix} \underline{ \hspace{30 pt} \pmb{ORGANIC ACID K_a pK_a} } \ \begin{matrix} Formic acid (HCOOH) & 1.8 imes 10^{-4} & 3.74 \ Acetic acid (CH_3COOH) & 1.8 imes 10^{-5} & 4.74 \ Propanoic acid (CH_3CH_2COOH) & 1.3 imes 10^{-5} & 4.89 \ Ascorbic acid (vitamin C) & 7.9 imes 10^{-5} & 4.10 \end{matrix} \end{matrix} [/latex]

ACID	$\pmb{K_a}$	ACID	$\pmb{K_a}$
Hydrofluoric acid (HF)	$3.5 \times 10^{-4}$	Polyprotic acids
Hydrocyanic acid (HCN)	$4.9 \times 10^{-10}$	Sulfuric acid
Ammonium ion $(NH_4 ^+)$	$5.6 \times 10^{-10}$	$H_2SO_4$	Large
		$HSO_4 ^-$	$1.2 \times 10^{-2}$
Organic acids		Phosphoric acid
Formic acid (HCOOH)	$1.8 \times 10^{-4}$	$H_3PO_4$	$7.5 \times 10^{-3}$
Acetic acid $(CH_3COOH)$	$1.8 \times 10^{-5}$	$H_2PO_4 ^-$	$6.2 \times 10^{-8}$
Propanoic acid $(CH_3CH_2COOH)$	$1.3 \times 10^{-5}$	$HPO_4 ^{2-}$	$2.2 \times 10^{-13}$
		Carbonic acid
Ascorbic acid (vitamin C)	$7.9 \times 10^{-5}$	$H_2CO_3$	$4.3 \times 10^{-7}$
		$HCO_3 ^-$	$5.6 \times 10^{-11}$

Question 10.12: Which of the organic acids in Table 10.2 would be the most a...

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