If a base is added to pure water, why does the [H 3 O + ] decrease?

Hello. In this problem, we are asked to explain why. Adding a base to pure water causes the concentration of hydro mines to decrease. We are provided with four possible answers. Answer a states the base reacts with hydro mines forming its conjugate acid. Thus reducing the concentration of acidic hydro mines. B the base reacts with water to form hydro mines. Thus reducing the concentration of hydrogen ions. C the base reacts with hydroxide ions to form more water molecules. And D the base has a higher affinity for hydris than water molecules. Leading to a decrease in the concentration of hydro mines. Recall that water can undergo auto organization. That then means that some water molecules act as an acid and donate a proton and others act as a base by accepting that proton, we then form hydro minds and hydroxide ions and establish an equilibrium. We can therefore write an equilibrium constant expression which is called the iron product, constant of water. It is the concentration of hydrogen ions times the concentration of hydroxid ions. And this is equal to one times 10 to minus 14 at 25 degrees Celsius. And so when we add a base, it will react with the acid component in water. So it will react with the hydro mines, hydronym ions, then will donate a proton to the hydroxid ions. And in doing so, then we will form two molecules of water in forming a molecule of water. Then we are consuming the hydro mines and we are generating then water which is the conjugate acid of our base hydroxide ions. And it is also true that the iron product, constant of water, it remains a constant as long as the temperature remains constant. So that then tells us if the hydroxid ion concentration goes up, the hydro ion concentration must go down. So looking at the answers that we have been provided, we have then answer a states that the base reacts with hydro mines forming its conjugate acid, thus reducing the concentration of acidic hydro mines. So this answer is correct answer B states the base reacts with water to form hydro mines, thus reducing the concentration of hydro mines. So that doesn't make sense that we form hydro mines and also at the same time, reduce the concentration of hydro mines. So that's incorrect. C states that the base reacts with hydroxide ions to form more water molecules. So that's incorrect. And answer D states the base has a higher affinity for hydron nines than water molecules leading to a decrease in the concentration of hydro mines. So the hydroxide reacts with the hydronic mines and thereby gains a proton, but it does not have greater affinity for the hydro nines than water molecules. So answer D is incorrect. So our correct answer is answer. A thanks for watching. Hope this helped.

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10. Acids and Bases

The pH Scale

GOB Chemistry

10. Acids and Bases

The pH Scale

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2:56 minutes

Problem 28

Textbook Question

If a base is added to pure water, why does the [H₃O⁺] decrease?

Verified step by step guidance

Understand that pure water undergoes autoionization, where water molecules react with each other to form hydronium ions (H₃O⁺) and hydroxide ions (OH⁻). This equilibrium is represented as:

{H (2 O)}_{2} \leftrightarrow {H (3 O)}_{+} {OH}_{-}

Recall that the product of the concentrations of H₃O⁺ and OH⁻ in water is constant at a given temperature, known as the ion product of water, K_w. At 25°C, K_w = 1.0 × 10⁻¹⁴, so [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴.

When a base is added to water, it increases the concentration of hydroxide ions (OH⁻) in the solution. This disrupts the equilibrium of the autoionization reaction.

To restore equilibrium, the system shifts according to Le Chatelier's Principle. The increase in OH⁻ concentration causes the reaction to shift to the left, reducing the concentration of H₃O⁺ ions.

As a result, the [H₃O⁺] decreases because the equilibrium adjusts to maintain the constant value of K_w, ensuring that [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25°C.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acid-Base Chemistry

Acid-base chemistry involves the study of substances that can donate protons (H⁺ ions) or accept protons. In this context, bases are substances that can accept protons, leading to a decrease in the concentration of hydronium ions (H₃O⁺) in solution. Understanding this concept is crucial for grasping how the addition of a base alters the balance of ions in water.

Equilibrium in Water

Pure water undergoes autoionization, where water molecules dissociate into H⁺ and OH⁻ ions, establishing a dynamic equilibrium. The concentration of H₃O⁺ in pure water is typically 1 x 10⁻⁷ M at 25°C. When a base is added, it shifts this equilibrium, reducing the concentration of H₃O⁺ as the base reacts with available protons.

pH Scale

The pH scale measures the acidity or basicity of a solution, with lower values indicating higher acidity (more H₃O⁺) and higher values indicating higher basicity (less H₃O⁺). When a base is introduced to water, the pH increases, reflecting a decrease in H₃O⁺ concentration. Understanding the pH scale is essential for interpreting changes in acidity when a base is added.

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