Calculate the number of H₃O⁺ ions in 1.0 mL of pure water at 25 °C.

Verified step by step guidance

1. Step 1: Understand that pure water at 25 °C is neutral, meaning the concentration of H₃O⁺ ions is equal to the concentration of OH^- ions.
2. Step 2: Recall that the concentration of H₃O⁺ ions in pure water at 25 °C is 1.0 × 10^-7 M.
3. Step 3: Convert the volume of water from milliliters to liters, since molarity is expressed in moles per liter. For 1.0 mL, this is 0.001 L.
4. Step 4: Use the formula for moles: moles = concentration (M) × volume (L). Substitute the known values to find the moles of H₃O⁺ ions.
5. Step 5: Convert the moles of H₃O⁺ ions to the number of ions using Avogadro's number (6.022 × 10²³ ions/mol).

Key Concepts

Ionization of Water

Water undergoes self-ionization, where two water molecules produce one hydroxide ion (OH^-) and one hydronium ion (H₃O⁺). This process is essential for understanding the concentration of H₃O⁺ ions in pure water, which occurs at a very low level.

pH and Concentration of Ions

The pH scale measures the acidity or basicity of a solution, defined as the negative logarithm of the H₃O⁺ concentration. In pure water at 25 °C, the pH is 7, indicating that the concentration of H₃O⁺ ions is 1.0 x 10^-7 M, which is crucial for calculating the number of ions in a given volume.

Avogadro's Number

Avogadro's number (6.022 x 10²³ entities/mol) is used to convert between moles of a substance and the number of individual particles, such as ions. To find the total number of H₃O⁺ ions in 1.0 mL of water, one must first calculate the number of moles in that volume and then use Avogadro's number to determine the total count of ions.