6D.11 Chemical Equations for salts

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Immi Lee - 1D
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Joined: Wed Sep 30, 2020 9:58 pm

6D.11 Chemical Equations for salts

Postby Immi Lee - 1D » Thu Dec 10, 2020 6:09 pm

Decide whether an aqueous solution of each of the following salts has a pH equal to, greater than, or less than 7. If pH>7 or pH<7, write a chemical equation to justify your answer:

a. NH4Br
b. Na2CO3
c. KF
d. KBr
e. AlCl3
f. Cu(NO3)2

I know how to determine whether the salt is acidic/neutral/basic, but after looking at the answer key I don't understand how to determine the chemical equation that justifies the answer.

Image

Could someone explain how the textbook got to those equations? Thanks!

Stuti Pradhan 2J
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Re: 6D.11 Chemical Equations for salts

Postby Stuti Pradhan 2J » Thu Dec 10, 2020 6:32 pm

You want to see what the effect of the salt would be on the aqueous solution, so you want to understand how the salt will interact with the water molecules. Within the salt, the conjugate base of any strong acid and the conjugate acid of any strong base will have no effect on the pH, so that ion can be ignored for this purpose. Whichever ion is not the conjugate base of a strong acid or the conjugate acid of a weak base will interact with the water. If this ion has a negative, it will attract the hydrogen from the water and leave behind an OH-, making the solution basic. If the ion has a positive charge, the water will attract the hydrogen, forming an H3O+ molecule and making the solution acidic.

For example, in part a) the Br- ion in NH4Br is the conjugate acid of HBr which is a strong acid, and NH4+ is the conjugate acid of NH3, which is a weak base. Therefore, the Br- has no effect on the pH of the solution. However, the NH4+ will interact with the water, losing its electron to water to create NH3 and H3O+.

Hope this helps!

Immi Lee - 1D
Posts: 93
Joined: Wed Sep 30, 2020 9:58 pm

Re: 6D.11 Chemical Equations for salts

Postby Immi Lee - 1D » Thu Dec 10, 2020 11:15 pm

Ohhh that makes sense thank you!!

Jacquelyn Challis 2B
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Re: 6D.11 Chemical Equations for salts

Postby Jacquelyn Challis 2B » Fri Dec 11, 2020 6:21 pm

How do you determine whether the salt is acid or basic?

RitaThomas_3G
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Joined: Wed Sep 30, 2020 9:40 pm

Re: 6D.11 Chemical Equations for salts

Postby RitaThomas_3G » Fri Dec 11, 2020 11:02 pm

To answer the question above ^, you can determine if a salt is an acid or base depending on the cation and anions that make up its composition, and the strength they would have in an acid or base. For example, in Na2S, the cation Na is derived of a strong base (NaOH). The anion S, however, is derived from a weak acid. Therefore, Na2S can be classified as basic.


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