Crime Busters B

samlan16
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Re: Crime Busters B

KS_1618 wrote:Hello all,

One more question about Crime Busters B. I was wondering - does anyone know what the pH of Sodium Acetate Anhydrous is? It seems as though it's 9, but as a buffer solution when mixed with acetic acid, it is between 3.6 and 5.6. It also doesn't react with HCl, but if it's a base or basic salt, it should. Please help.

Thanks.
Sodium acetate is a basic salt, which means that when it is dissolved in water, it allows the water to form hydroxide species (i.e. $OH^-$) that are stronger than a weak conjugate acid (acetic acid). Yes, the pH is supposed to be around 8 or 9.

The reason solid sodium acetate will not react with HCl is because $CH_3COONa$ is already an electrically stable salt when solid. It must be dissolved to form hydroxide ions with the water, and those rather than the sodium and acetate ions will react the acid. You won't be able to see it because 1) you won't have an indicator and 2) each step does not go to completion, meaning that only a fraction of the reactants will be used.

Long story short, here are the reactions that occur during the dissolution of CH3COONa:

$\ch{CH_3COONa_{(s)} \rightleftharpoons Na^+_{(aq)} + CH_3COO^-_{(aq)}}$
$\ch{CH_3COO^-_{(aq)} + H_2O_{(l)}\rightleftharpoons CH_3COOH_{(aq)} + OH^-_{(aq)}}$

And when HCl is added:

$\ch{HCl_{(aq)} + OH^-_{(aq)} \rightleftharpoons H_2O_{(aq)} + Cl^-_{(aq)}}$

With respect to the buffer, the low pH is due to a large quantity of acetic acid mixed with a much smaller quantity of sodium acetate's constituent ions. Don't pay attention to the buffer if you want the pH of the salt itself.
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KS_1618
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Re: Crime Busters B

Thanks - really helped a lot.

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