Log out Go to App
Go to App

Learning Materials

Features

Discover

Chapter 12: Problem 3

Explain what happens to (a) the \(\mathrm{pH}\) of a solution of phosphoric acid after the addition of solid sodium dihydrogen phosphate; (b) the percentage deprotonation of \(\mathrm{HCN}\) in a hydrocyanic acid solution after the addition of hydrobromic acid; (c) the concentration of \(\mathrm{H}_{3} \mathrm{O}^{+}\)ions when pyridinium chloride is added to an aqueous solution of the base pyridine.

Short Answer

Expert verified
After adding NaH2PO4 to a phosphoric acid solution, the pH will increase. Adding HBr to a hydrocyanic acid solution decreases the percentage deprotonation of HCN. Pyridinium chloride added to a pyridine solution increases the concentration of H3O+ ions, thereby lowering pH.

Step by step solution

01

Understanding the Effect on pH

When solid sodium dihydrogen phosphate (NaH2PO4) is added to a solution of phosphoric acid (H3PO4), it acts as a source of common ions, namely H2PO4-. This increases the concentration of H2PO4- ions shifting the equilibrium position of the dissociation reaction of H3PO4 to the left due to Le Chatelier's principle. This results in less H+ ion production and thus an increase in pH.
02

Evaluating the Percentage Deprotonation of HCN

The addition of hydrobromic acid (HBr) to a solution of hydrocyanic acid (HCN) increases the concentration of H+ ions in the solution since HBr is a strong acid. This increased concentration of H+ will shift the equilibrium of the weak acid (HCN) to the left, resulting in less dissociation of HCN into H+ and CN-. The percentage deprotonation of HCN thus decreases.
03

Effect on the Concentration of Hydronium Ions

When pyridinium chloride is added to an aqueous solution of the base pyridine, the pyridinium ion, C5H5NH+, which is the conjugate acid of pyridine, releases H+ ions into the solution resulting in an increased concentration of H3O+ ions. This process effectively lowers the pH of the solution.

Unlock Step-by-Step Solutions & Ace Your Exams!

  • Full Textbook Solutions

    Get detailed explanations and key concepts

  • Unlimited Al creation

    Al flashcards, explanations, exams and more...

  • Ads-free access

    To over 500 millions flashcards

  • Money-back guarantee

    We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!

Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Le Chatelier's Principle
Le Chatelier's principle is a fundamental concept in chemical equilibrium that predicts how a system at equilibrium reacts to disturbances.
This principle states that if an external change is applied to a system in equilibrium, the system will adjust itself to counteract that change and establish a new equilibrium.
  • When the concentration of a reactant is increased, the system shifts to produce more products.
  • If the concentration of a product is increased, the system shifts to produce more reactants.
  • Changes in pressure and temperature can also cause the system to shift in a direction that counteracts the change.
Applying this principle to the exercise, when sodium dihydrogen phosphate is added to phosphoric acid, the presence of a common ion causes the equilibrium to shift to the left, reducing the concentration of hydrogen ions and increasing the pH.
pH Level
The pH level is a measure of the acidity or basicity of a solution.
It is defined as the negative logarithm of the hydrogen ion concentration: \( pH = -\text{log}[H^+] \).
  • A pH lower than 7 indicates an acidic solution, while a pH higher than 7 indicates a basic solution.
  • The pH scale is logarithmic, which means each whole pH value below 7 is ten times more acidic than the next higher value.
In our exercise, the addition of sodium dihydrogen phosphate to phosphoric acid results in an increased pH due to the suppression of hydrogen ion generation.
Deprotonation
Deprotonation is the removal of a proton (H+) from a molecule, forming its conjugate base.
In an acidic solution, deprotonation is represented by the dissociation of the acid to yield hydrogen ions and its conjugate base:
  • For weak acids, such as hydrocyanic acid (HCN), the percentage deprotonation refers to the proportion of acid molecules that have donated a proton.
  • This percentage is influenced by the concentration of hydrogen ions in the solution, with a higher concentration leading to less deprotonation.
Accordingly, when a strong acid like hydrobromic acid is added to an HCN solution, it increases the hydrogen ion concentration, thus forcing HCN to remain largely undissociated, and decreasing the percentage deprotonation.
Hydrogen Ion Concentration
Hydrogen ion concentration is a crucial determinant of a solution's pH.
It is denoted by \( [H^+] \) and refers to the number of moles of hydrogen ions per liter of solution. An increase in \( [H^+] \) makes a solution more acidic, while a decrease has the opposite effect.
  • The hydrogen ion concentration is a central aspect in understanding acid-base reactions, equilibrium, and pH calculations.
  • For instance, the addition of pyridinium chloride to a pyridine solution increases \( [H^+] \) due to the release of hydrogen ions from the pyridinium ion, which leads to a decrease in the pH, highlighting the dynamic nature of equilibrium in acid-base reactions.
Conjugate Acid-Base Pair
A conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton.
  • The acid donates a proton to become its conjugate base, while the base accepts a proton to become its conjugate acid.
  • This pair is essential in understanding the behavior of acids and bases in solution and is closely tied to the concepts of equilibrium and pH.
When pyridine reacts with water, it forms its conjugate acid, pyridinium. The addition of a salt like pyridinium chloride supplies more of the conjugate acid, which can release H+ ions, thereby affecting the pH of the solution as observed in the exercise scenario.

One App. One Place for Learning.

All the tools & learning materials you need for study success - in one app.

Get started for free

Most popular questions from this chapter

The molarity of \(\mathrm{CrO}_{4}^{2-}\) in a saturated \(\mathrm{Tl}_{2} \mathrm{CrO}_{4}\) solution is \(6.3 \times 10^{-5} \mathrm{~mol} \cdot \mathrm{L}^{-1}\). What is the \(K_{\mathrm{sp}}\) of \(\mathrm{Tl}_{2} \mathrm{CrO}_{4}\) ?

A buffer solution of volume \(100.0 \mathrm{~mL}\) is \(0.100 \mathrm{M}\) \(\mathrm{CH}_{3} \mathrm{COOH}(\mathrm{aq})\) and \(0.100 \mathrm{M} \mathrm{NaCH} \mathrm{CO}_{2}(\mathrm{aq})\). (a) What are the \(\mathrm{pH}\) and the pH change resulting from the addition of \(10.0 \mathrm{~mL}\) of \(0.950 \mathrm{M} \mathrm{NaOH}(\mathrm{aq})\) to the buffer solution? (b) What are the \(\mathrm{pH}\) and the \(\mathrm{pH}\) change resulting from the addition of \(20.0 \mathrm{~mL}\) of \(0.100 \mathrm{M} \mathrm{HNO}_{3}(\mathrm{aq})\) to the initial buffer solution?

What volume of \(0.123 \mathrm{M} \mathrm{NaOH}(\mathrm{aq})\) must be added to \(125 \mathrm{~mL}\) of \(0.197 \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{3}(\mathrm{aq})\) to reach (a) the first stoichiometric point; (b) the second stoichiometric point?

The molar solubility of cerium(III) hydroxide, \(\mathrm{Ce}(\mathrm{OH})_{3}\), is \(5.2 \times 10^{-6} \mathrm{~mol} \cdot \mathrm{L}^{-1}\). What is the \(K_{\mathrm{sp}}\) of cerium(III) hydroxide? For the purpose of this calculation, ignore the autoprotolysis of water.

\(12.97\) In an attempt to determine the concentration of sulfur dioxide in the air near a power plant, two students set up a bubbler that passes air through \(50.00 \mathrm{~mL}\) of \(1.0 \times 10^{-4} \mathrm{M}\) \(\mathrm{NaOH}(\mathrm{aq})\). The temperature is \(22^{\circ} \mathrm{C}\), and the atmospheric pressure 753 Torr. The air is pumped for \(2.5 \mathrm{~h}\) at a flow rate of \(3.0 \mathrm{~L} \cdot \mathrm{h}^{-1}\). The students then returned to the laboratory and titrated the solution with \(1.5 \times 10^{-4} \mathrm{M} \mathrm{HCl}(\mathrm{aq})\) with phenolphthalein indicator to see how much \(\mathrm{NaOH}\) was left unreacted. They found that \(30.2 \mathrm{~mL}\) of \(\mathrm{HCl}(\mathrm{aq})\) was required to reach the stoichiometric point. (a) Write the balanced chemical equation for the reaction of \(\mathrm{SO}_{2}\) and water. (b) What amount of \(\mathrm{NaOH}\) (in mol) had reacted with the \(\mathrm{SO}_{2}\) ? (c) What was the concentration of sulfur dioxide in the air, in parts per million?
See all solutions

Recommended explanations on Chemistry Textbooks

The Earths Atmosphere

Read Explanation

Kinetics

Read Explanation

Chemistry Branches

Read Explanation

Physical Chemistry

Read Explanation

Inorganic Chemistry

Read Explanation

Nuclear Chemistry

Read Explanation
View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Study anywhere. Anytime. Across all devices.

Sign-up for free