pH of AlCl3 (aq)?

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pH of AlCl3 (aq)?

Postby aisteles1G » Wed Jan 16, 2019 6:59 pm

I dont't understand how to find the ph of such a solution since theres no hydrogen in it? At first I was thinking it would be 7 because maybe it's neutral, anion and a cation with balanced charges, but it's not. This is question 6D.15 in the 7th edition book. The solutions manual puts in H20 in place of Cl3 I think but that completely threw me off, any help would be greatly appreciated, thanks!

Sierra Cheslick 2B
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Re: pH of AlCl3 (aq)?

Postby Sierra Cheslick 2B » Wed Jan 16, 2019 7:39 pm

This is a salt, and it dissociates in water. Since Cl- is the conjugate base of a strong base, it is very weak and therefore considered to not affect pH and can be left out of the equation. Al3+, however, can pull water molecules towards it and form coordinate covalent bonds, since it is a small and highly charged cation. These coordinate covalent bonds weaken the the O-H bonds by pulling electrons towards the Al, and allow hydrogens to detach from the water molecules. This forms H3O+, meaning Al3+ acts as a weak acid. To solve for the pH, set up and ICE box with the concentration of Al3+ minus x, and the initial concentration of H3O+ (0 M) plus x. Then solve for x using the Ka value for Al3+ (1.4 x 10^-5)

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