chem module 10
Name: __________ _________________________
CHEM 1212 Module 10 Reading Guide
Salts
Even if we mix stoichiometrically equivalent quantities of acid and base, we may find that the resulting solution is not __________. Many salts have acidic or basic properties. The word salt refers to __________compounds that do not contain __________ as the cation or __________ as the anion. This occurs if the anion or cation in the salt is the conjugate of a __________or a weak base. Certain __________cations that have a high charge density (a high positive charge combined with a small ionic diameter) can also act as __________.
The strength of an acid or base is inversely proportional to the strength of its conjugate base or conjugate acid. If the salt is made up of a cation from a __________and an __________from a strong acid, it will have __________acid–base character. Cations that are conjugate acids of weak bases are __________. Anions that are conjugate bases of weak acids are __________. The anion of monoprotic strong acids and the group 1 and 2 cations have no acid–base properties, so salts made up of these ions do not affect the solution __________.
The pH of a salt can be calculated if you know its __________and the Ka or Kb for the acidic or basic ions that make up the salt. When the conjugate acid and the conjugate base are of __________strengths, the solution can be either acidic or __________, depending on the relative strengths of the two conjugates. To predict whether a particular combination will be acidic or basic, tabulated K values of the conjugates must be compared. Whichever ion has __________acid or base properties will dominate, and will drive the pH of the solution in the corresponding direction. The strength of a weak acid and that of its conjugate base are related through the __________ equilibrium, so if you know the value of Ka, then it is possible to calculate Kb for the conjugate base. Remember this changes with __________.
Practice Problems
66. Which of the following 0.100 M solutions are acidic, which are basic, and which are neutral?
a. NaC2H3O2
b. K2S
c. NaNO2
d. LiClO3
e. NH4ClO3
71. If the Ka for a weak acid, HA, is 4.5 × 10−6, what is the Kb for its conjugate base, A−?
73. Calculate the pH of 0.0470 M LiBrO. Ka of HBrO = 2.8 x 10-9
Buffers
Buffer solutions __________changes in __________by neutralizing added acid or base. A buffer solution contains both a weak acid and its __________. A buffer is made with significant quantities of both HA and its conjugate base, A−, allowing it to __________in either direction according to __________principle when either acid or base from another source is added. The pH of a buffer solution does not change appreciably when the solution is __________.
When a weak electrolyte is combined with a __________electrolyte that contains a __________ion, the ionization of the weak electrolyte is __________. This phenomenon is known as the __________ effect and can be explained via Le Châtelier’s principle. Because of the common-ion effect, buffers __________very little, making the starting concentrations of the conjugate pair a very good approximation for the __________concentrations.
The effectiveness of a buffer solution is based on two factors: buffer capacity and effective pH range. __________is the ability of a buffer solution to resist a change in pH when an acid or base is added. A buffer that has a __________concentration of conjugate acid and conjugate base will resist a pH change more strongly than a buffer with a __________concentration of conjugate acid and conjugate base.
Buffers resist a pH change most effectively when the concentrations of the weak acid and conjugate base components of the buffer are __________, when the conjugate acid and base concentrations are appreciably high compared to the conjugate acid’s __________ value, and when the pH of the solution is ±1 unit from the acid’s __________.
Practice Problems
4. Determine which of the following compounds would form a buffer solution when dissolved in water:
a. HCl and NaCl
b. HClO and NaCl
c. HClO and NaClO
d. CO and CO2
5. Select the statement or statements that are true about buffer solutions:
a. Buffer solutions consist of a strong acid and its conjugate base.
b. The pH of a buffer solution never changes, even when a large amount of strong acid or base is added.
c. Buffer solutions resist a change in pH by shifting the position of their equilibria to counteract the addition of H3O+ or OH−.
13. Which of the following weak acids would be suitable for making a buffer at pH = 3.50?
a. chlorous acid (Ka = 1.1 × 10−2)
b. formic acid (Ka = 1.8 × 10−4)
c. hydrofluoric acid (Ka = 6.3 × 10−4)
d. hypobromous acid (Ka = 2.8 × 10−9)
Henderson-Hasselbalch Equation
The __________equation relates the pH of a buffer solution to the pKa of the weak acid and the concentrations of its acidic and basic components.
The Henderson–Hasselbalch equation uses the assumption that the equilibrium concentrations are approximately equal to the __________concentrations. The Henderson–Hasselbalch equation can also be used to determine the relative concentrations of HA and A− that are necessary to make a buffer solution with a specific __________ value.
Practice Problems
9. Calculate the pH of a buffer that contains 0.390 M hydrazine, N2H4 (Kb = 1.3 × 10−6), with 0.264 M hydrazinium ion, N2H5+.
10. Calculate the ratio of conjugate base to weak acid, [A−]/[HA], needed to make a buffer at pH = 5.00 if the pKa of the acid is 4.20.
Text segments from OpenStax, Chemistry. OpenStax CNX. Sep 15, 2020 http://cnx.org/contents/85abf193-2bd2-4908-8563-90b8a7ac8df6@13.1.; White, J. et al. Interactive General Chemistry. MacMillan, NY. 2019.; Chemistry LibreTexts, Libretexts, [online] Available from: https://chem.libretexts.org/Courses/Heartland_Community_College/HCC:_Chem_161/5:_Thermochemistry/5.7:_Enthalpy_of_Formation. Reading guide style Adapted from "Ionic equations Study Guide" by Montgomery College is licensed under CC BY 4.0 Document not to be reposted on the internet without express permission.