3.1.3 - pH Calculation for Strong Acids/Bases
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Understanding Strong Acids
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Today, we're discussing strong acids. Can anyone tell me what a strong acid is?
Is it an acid that completely dissociates in water?
Exactly! When a strong acid like HCl is dissolved in water, it fully dissociates into HβΊ and Clβ» ions. So how do we calculate the pH?
Is it the concentration of HβΊ ions?
Yes, it is! The formula is pH = -logββ[HβΊ]. So, if you have a 0.100 M HCl solution, you would calculate pH as follows: pH equals -logββ(0.100). What result do you get?
The pH would be 1.00.
Correct! Now letβs move on to strong bases.
Calculating pH for Strong Bases
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Now, let's talk about strong bases like NaOH. Who can tell me how a strong base behaves in water?
It dissociates completely to produce OHβ» ions.
Correct! If we have a 0.050 M NaOH solution, the concentration of OHβ» is also 0.050 M. To find pH, we need pOH first. How do we calculate that?
pOH = -logββ(0.050)?
Exactly! And once you have pOH, remember that pH + pOH = 14. So whatβs the pOH for 0.050 M NaOH?
It's 1.30, so pH would be 14 - 1.30, which equals 12.70.
Excellent! You all are grasping this quickly.
Practical Examples of pH Calculation
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Let's apply what we've learned! Suppose we dissolve 0.250 mol of HCl in 1.00 L of water. What will the pH be?
The concentration of HβΊ would be 0.250 M, so pH = -logββ(0.250) = 0.60.
Correct! Now, how about if we dissolve 0.0200 mol of Ca(OH)β in 500 mL of water?
Ca(OH)β gives us 2 x 0.0400 M of OHβ», which is 0.0800 M. So pOH = -logββ(0.0800) = 1.10.
Right! So what is our final pH?
pH = 14.00 - 1.10 equals 12.90.
Great job! These are the steps to calculating the pH of strong acids and bases.
Introduction & Overview
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Quick Overview
Standard
The pH calculation for strong acids and bases involves understanding complete dissociation in solutions. This section explains the principles behind calculating pH and pOH, using straightforward formulas and examples to illustrate the method for both acids and bases.
Detailed
pH Calculation for Strong Acids/Bases
In this section, we explore how to accurately calculate the pH of solutions containing strong acids and bases. Strong acids, such as hydrochloric acid (HCl) and sulfuric acid (HβSOβ), fully dissociate in water, meaning they release hydrogen ions (HβΊ) completely. Conversely, strong bases, like sodium hydroxide (NaOH) and calcium hydroxide (Ca(OH)β), dissociate to yield hydroxide ions (OHβ»).
Understanding that for a solution of a strong acid with concentration C, the concentration of HβΊ is equal to C allows us to use the formula:
- pH = -logββ[HβΊ]
When dealing with strong bases, similar logic applies; the concentration of OHβ» equals C. The relationship between pH and pOH is defined by:
- pH + pOH = 14 (at 25Β°C)
Examples provided in this section illustrate these calculations. For instance, dissolving 0.250 mol of HCl in 1 liter of water leads to:
- [HβΊ] = 0.250 M, therefore pH = -logββ(0.250) = 0.60.
For strong bases like Ca(OH)β, which produces two hydroxide ions per formula unit, careful calculation is necessary to determine [OHβ»]. Calculating the pH involves steps that ensure the dissociative action is accounted for, leading to accurate pH comprehension for solution chemistry.
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Understanding pH for Strong Acids
Chapter 1 of 4
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Chapter Content
For a solution of a strong acid with concentration C, [H plus] = C (assuming negligible contribution from water if C > 10β»βΆ).
Detailed Explanation
When we talk about strong acids, they are substances that dissociate completely in water to give hydrogen ions (H plus). This means that the concentration of hydrogen ions in the solution is equal to the concentration of the acid itself (C). If the concentration of the acid is greater than 10β»βΆ M, we can fairly ignore the contribution of hydrogen ions from the self-ionization of water, simplifying our calculations.
Examples & Analogies
Imagine you are making lemonade. If you add a specific amount of lemon juice (strong acid), all of that juice mixes into the water (solvent) completely, and the sour taste you experience corresponds directly to how much lemon juice you added. This direct relationship is what occurs in strong acid solutions with respect to pH.
Calculating pH for Strong Acids: Example 1
Chapter 2 of 4
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Chapter Content
Example 1: A solution is prepared by dissolving 0.250 mol of HCl in 1.00 L of water.
[H plus] = 0.250 M β pH = β logββ (0.250) = 0.60.
Detailed Explanation
In this example, we have dissolved 0.250 mol of hydrochloric acid (HCl) in 1 liter of water. Since it's a strong acid, it completely dissociates to produce 0.250 M of hydrogen ions in solution. To calculate the pH, we use the formula pH = -logββ([H plus]). Plugging in 0.250 M, we find that the pH is 0.60, indicating a very acidic solution.
Examples & Analogies
Think of pouring lemon juice (HCl) into a glass of water. If you pour a certain amount into one liter of water, it would be very sour, and the acidity (pH) indicates just how sour it is. A pH of 0.60 means itβs quite sour.
Calculating pH for Strong Bases
Chapter 3 of 4
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Chapter Content
For a solution of a strong base with concentration C, [OH minus] = C (again, ignoring water).
Detailed Explanation
Strong bases also dissociate completely in water, producing hydroxide ions (OH minus). Similar to strong acids, the concentration of hydroxide ions in a strong base solution is equal to the initial concentration of the base. When calculating, we again ignore the contribution from water as long as this concentration is above 10β»βΆ M.
Examples & Analogies
Imagine washing your hands with soap that contains a strong base like sodium hydroxide (NaOH). When you mix it with water, the soap completely dissolves, increasing the cleanliness (alkalinity) of the water based on how much soap you used. Here, the pH increase indicates how 'slippery' or basic that solution is.
Calculating pH for Strong Bases: Example 2
Chapter 4 of 4
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Chapter Content
Example 2: A solution is prepared by dissolving 0.0200 mol of Ca(OH)β in 500 mL of water. Ca(OH)β dissociates into CaΒ² plus + 2 OH minus.
Moles of Ca(OH)β = 0.0200 mol, volume = 0.500 L β 0.0400 M Ca(OH)β.
[OH minus] = 2 Γ 0.0400 M = 0.0800 M.
pOH = β logββ (0.0800) = 1.10 β pH = 14.00 β 1.10 = 12.90.
Detailed Explanation
In this case, we dissolved 0.0200 mol of calcium hydroxide (Ca(OH)β) in 500 mL of water. First, we convert the volume to liters to find the molarity of the solution. Since Ca(OH)β produces 2 hydroxide ions for each formula unit, the resulting hydroxide ion concentration is 0.0800 M. To find the pOH, we use pOH = -logββ([OH minus]), which gives us 1.10. Finally, using the relationship pH + pOH = 14, we calculate the pH to be 12.90, indicating a very basic solution.
Examples & Analogies
Consider using a strong drain cleaner that contains calcium hydroxide. When you pour it into water, the strong base dissociates fully and makes the water very slippery and basic, similar to how larger amounts of soap would. The pH level reflects how basic that water is, and here at 12.90, itβs quite alkaline.
Key Concepts
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pH and pOH definitions: pH measures acidity, and pOH measures basicity.
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Strong acids fully dissociate: Understanding the complete dissociation of strong acids into HβΊ ions.
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pH calculation for strong acids: pH is directly calculated using the concentration of HβΊ ions.
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Strong bases fully dissociate: Similar to strong acids, but yielding OHβ» ions instead.
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Calculation of pOH from concentration: Using the formula pOH = -logββ[OHβ»].
Examples & Applications
Example 1: Dissolving 0.250 mol of HCl in 1.00 L of water leads to a concentration of HβΊ of 0.250 M and a pH of 0.60.
Example 2: Dissolving 0.0200 mol of Ca(OH)β in 500 mL of water results in [OHβ»] = 0.0800 M, leading to a pH of 12.90.
Memory Aids
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Rhymes
If HCl's the acid that you see, the pH will drop just like a tree.
Stories
Imagine at a pool party, everyone jumps in. Strong acids make a splash by fully dissociating in water, just like how the whole pool fills up when each person jumps in.
Memory Tools
Remember to use 'pH is maybe' - use pH = -logββ[HβΊ] to find out how acidic things be.
Acronyms
BASIC
= Dissolving Bases (like NaOH) Always Splits Into Cations and OHβ».
Flash Cards
Glossary
- Strong Acid
An acid that completely dissociates in solution, yielding HβΊ ions.
- Strong Base
A base that completely dissociates in solution, yielding OHβ» ions.
- pH
A measure of the acidity or basicity of a solution, calculated as pH = -logββ[HβΊ].
- pOH
A measure of the concentration of hydroxide ions in a solution, calculated as pOH = -logββ[OHβ»].
- Concentration
The amount of a substance in a specified volume of solution.
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