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Solving Acid and Base Equilibrium

In this detailed overview of acid-base chemistry, we explore the concepts of pH and pOH as logarithmic scales representing hydronium and hydroxide ion concentrations. The relationship between pH, pOH, and the autoionization of water is examined, along with the use of the Kw equation for equilibrium calculations. We will demonstrate how to solve for pH and pOH in strong and weak acid/base solutions through practical examples, including the application of ICE charts and the assumption method for calculating concentrations.

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Solving Acid and Base Equilibrium

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  1. Solving Acid and Base Equilibrium SCH4U0

  2. Power of Hydrogen • In acidic and basic solutions, we gauge the level of acidity using the pH scale • pH = power of hydrogen • The pH scale is a logarithmic scale of hydronium concentration in a solution • Because it is a negative logarithm, low numbers represent high concentrations of hydronium

  3. Power of Hydrogen Basic Low [ Acidic High • We can define a similar scale in terms of hydroxide concentration called pOH • The “p” means “negative logarithm of” • pOH is similar to pH, low pOH means high hydroxide concentration

  4. pOH Scale • Both the pH and pOH scale are logarithmic, meaning that each unit represents a 10x difference from the previous unit • Eg: • A solution with pH 1 contains 10x more hydronium than one with pH 2 • A solution with pOH 1 contains 10x more hydroxide than one with pH 2

  5. Autoionization of Water • The hydroxide and hydronium concentrations in water are linked together due to the autoionization of water • When either hydronium or hydroxide is added to water, the equilibrium shifts left, reducing the other concentration

  6. Kw Equation • We can use the Kw equation to relate pH and pOH • Take the negative log of both sides

  7. Solving pH and pOH • Lets use our knowledge of acid/base strength to determine the pH and pOH of certain systems • Eg: What is the pH, pOH, and hydroxide concentration in a 0.050M solution? • Nitric acid is a strong acid, so it dissociates 100%

  8. Solving pH and pOH • Using this, we can easily calculate pH and pOH

  9. Weak Acids/Bases • Solving the pH/pOH becomes a little more interesting when we consider weak acids and bases. • Eg: What is the pH, pOH, and hydroxide concentration in a 0.10 M ascorbic acid () solution? • What is the dissociation equation for this compound? • If we look up the Ka/Kb, ascorbic acid is a decent weak acid but a terrible base • To determine pH, pOH, and the hydroxide concentration we will need to determine the hydronium concentration by solving the EQ.

  10. Sample Problem • Fill in the ICE chart

  11. Sample Problem • To solve we select the most useful method • Try the assumption • If it doesn’t work, solve the quadratic equation The assumption is valid

  12. Solving pH and pOH • Using this, we can easily calculate pH and pOH

  13. Practice • Try the following problem yourself • Eg: What is the pH, pOH, and hydronium concentration in a 0.40 M hydrazine () solution? • What is the dissociation equation for this compound? • If we look up the Ka/Kb, hydrazine is a decent weak base but a terrible acid

  14. Sample Problem • Fill in the ICE chart

  15. Sample Problem • To solve we select the most useful method • Try the assumption • If it doesn’t work, solve the quadratic equation The assumption is valid

  16. Solving pH and pOH • Using this, we can easily calculate pH and pOH

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