## HW 6D11

Moderators: Chem_Mod, Chem_Admin

AChoudhry_1L
Posts: 55
Joined: Sat Sep 07, 2019 12:17 am

### HW 6D11

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

How would you determine the charge on the reactants in c, e, and f? Is it the charge that tells us whether something is an acid or a base?

Diana_Diep2I
Posts: 110
Joined: Sat Aug 17, 2019 12:17 am

### Re: HW 6D11

In c), F- is the anion of a weak acid (HF). In lecture today we learned that the conjugate base of a weak acid will raise the pH of the solution by removing protons from water to generate OH-. As a result, the pH will be >7.
e) Al3+ is the transition metal that is hydrated and has strong e- withdrawing power that causes electrons to be drawn to it. As a result, the -OH bond is broken and it gives off an H+ proton. Therefore the pH is <7.
f) Similarly to the above problem e, Cu2+ is one of the small highly charged metal cations that acts as a lewis acid. H+ will be broken off and this will cause the pH to be <7.

I'm not sure about the charges, but I pay attention to the individual anion and cation to see if they are the conjugate acid/base of a strong or weak acid/base.

Diana_Diep2I
Posts: 110
Joined: Sat Aug 17, 2019 12:17 am

### Re: HW 6D11

Sorry I forgot to mention that you should look for the strong conjugate acids/bases that come from weak acids/bases. Like for example in a) NH4Br, Br- is a weak conjugate base of the strong acid HBr so it doesn't affect the pH and you shouldn't pay attention to that. NH4+ on the other hand, is a strong conjugate acid that comes from the weak base NH3 so you should use that instead.

As for the compounds with the highly charged metal cations, I think you should pay attention to the cations and know that whenever a compound has something like Cu2+ or Al3+, the pH will be more acidic when the metal cation becomes hydrated.

Return to “Calculating pH or pOH for Strong & Weak Acids & Bases”

### Who is online

Users browsing this forum: No registered users and 1 guest