Indicate whether the pH increases, decreases, or remains the same when each of the following is added.
(CH3NH3)Cl to a solution of CH3NH2
pyridinium nitrate, (C5H5NH)(NO3) to a solution of pyridine, C5H5N
sodium formate to a solution of formic acid
how do you know these?
You recognize what the hydrolysis products are; alternately, recognize the kind of a salt you have. I use the latter and my instructions may take a few liberties with actual chemical processes; however, they always give the correct answer.
strong acid is SA; example HCl, HBr,etc.
weak acid is WA; example acetic acid
strong base is SB; KOH, NaOH, etc
weak base is WB; NH3, organic NH2 etc.
CH3NH3Cl is the salt of CH3NH2&HCl. That's a SA and WB. So it is acid.
sodium formate is the salt of formic acid and NaOH. NaOH SB, formic acid is WA. So it is basic
SA + SB = neutral
SA + WB = acid
WA + SB = basic
WA + WB = acid, basic, neutral and i all depends upon which which has the larger Ka or Kb. If Ka is larger it is acid; if Kb is larger it is basic. If Ka = Kb it is neutral.
so how do i know if the ph is changing?
To determine whether the pH increases, decreases, or remains the same when certain substances are added to a solution, we need to consider the nature of the substances being added and their effect on the acidity or alkalinity of the solution. Here's an explanation of each scenario:
1. (CH3NH3)Cl to a solution of CH3NH2:
In this case, we have a salt (CH3NH3)Cl, which dissociates into its ions, CH3NH3+ and Cl-. CH3NH2 is a weak base, and when the salt is added to the solution, the concentration of CH3NH2 increases. Since the salt is formed from a weak base, the concentration of the conjugate acid (CH3NH3+) increases as well. As a result, the pH of the solution decreases slightly, making it more acidic.
2. Pyridinium nitrate, (C5H5NH)(NO3) to a solution of pyridine, C5H5N:
In this case, pyridinium nitrate is a salt formed from the pyridinium ion (C5H5NH+). Pyridine (C5H5N) is a weak base and can accept protons to form the pyridinium ion. When the salt is added to the solution of pyridine, the concentration of the pyridinium ion increases. Since the pyridinium ion is the conjugate acid of pyridine, the pH of the solution increases slightly, making it more alkaline.
3. Sodium formate to a solution of formic acid:
Formic acid (HCOOH) is a weak acid that partially dissociates into formate ions (HCOO-). When sodium formate (NaHCOO) is added to the formic acid solution, it dissociates completely into Na+ and HCOO- ions. The introduction of sodium formate increases the concentration of formate ions, which are formed from the conjugate base of formic acid. Consequently, the pH of the solution increases slightly, making it more alkaline.
To come to these conclusions, knowledge of acid-base chemistry and the behavior of weak acids and bases is necessary. Additionally, understanding the concept of conjugate acids and bases is crucial in determining the pH changes when salts are added to solutions.