Which of the following statements is true about the pH of a neutral solution at 25°C?

Question

Examoogle · Accepted Answer

The correct answer is Option D: The pH is equal to 7.. In aqueous solutions at 25°C, a neutral solution has an equal concentration of hydrogen ions ($H^+$) and hydroxide ions ($OH^-$). This balance results in a pH value of exactly 7.

Step-by-Step Solution & Concepts:
In aqueous solutions at 25°C, a neutral solution has an equal concentration of hydrogen ions ($H^+$) and hydroxide ions ($OH^-$). This balance results in a pH value of exactly 7.

Working Principle: The pH scale is defined as the negative logarithm of the hydronium ion concentration, $pH = -\log_{10}[H^+]$. In pure water at 25°C, the auto-ionization constant of water ($K_w$) is $1.0 	imes 10^{-14}$. Since $[H^+] = [OH^-]$, it follows that $[H^+] = \sqrt{K_w} = 1.0 	imes 10^{-7}$ M, leading to $pH = -\log(10^{-7}) = 7$.

Key Formulas: $pH = -\log_{10}[H^+]$ — Definition of pH; $K_w = [H^+][OH^-] = 10^{-14}$ — Auto-ionization constant of water at 25°C

Key Points: pH; pH > 7 indicates a basic/alkaline solution (high $[OH^-]$).; The pH scale is logarithmic, meaning a change of 1 pH unit represents a 10-fold change in $[H^+]$ concentration.

Applications: Testing purity of distilled water; Biological homeostasis in human blood

Additional Info: Pure water at room temperature is the standard reference for a neutral pH.. Option A is incorrect because pH > 7 represents basicity.. Option C is incorrect because pH. Option B is incorrect because a neutral solution is defined specifically by the value 7.

Answer: D is correct — At 25°C, a neutral solution has an equal concentration of hydronium and hydroxide ions, which corresponds to a pH of exactly 7.

Exam Tip: Remember that pH is temperature-dependent; the neutral point of 7 is only accurate at 25°C. At higher temperatures, the neutral pH value actually shifts slightly lower.

Examoogle · Answer

Option A: The pH is greater than 7.

Examoogle · Answer

Option B: The pH is not equal to 7.

Examoogle · Answer

Option C: The pH is less than 7.

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