Guess how many water molecules self-ionize in one liter of water! a) 7 moles b) 1 mole c) 10,000,000 moles d) 0.0000001 moles
The Correct Answer and Explanation is :
The correct answer is d) 0.0000001 moles.
Water self-ionizes, meaning that a small number of water molecules dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻) at any given time. This process is a reversible reaction represented by the equation:
[
2H_2O \rightleftharpoons H_3O^+ + OH^-
]
At 25°C, the equilibrium concentration of hydrogen ions (H⁺) or hydroxide ions (OH⁻) in pure water is 1 × 10⁻⁷ moles per liter. This corresponds to a very small fraction of the total water molecules present.
Calculation:
A mole is defined as (6.022 \times 10^{23}) molecules. In one liter of water, there are approximately:
[
55.5 \, \text{moles of H}_2O
]
This is because the molar mass of water is about 18.015 g/mol, and 1 liter of water weighs approximately 1000 grams, so:
[
\frac{1000 \, \text{grams}}{18.015 \, \text{g/mol}} \approx 55.5 \, \text{moles of H}_2O
]
For the self-ionization, the concentration of ions is (1 \times 10^{-7} \, \text{moles/L}). Thus, in one liter of water, the number of ions produced by self-ionization would be:
[
1 \times 10^{-7} \, \text{moles/L} \times 1 \, \text{L} = 1 \times 10^{-7} \, \text{moles of H}^+ \text{ and OH}^-
]
This is equivalent to 0.0000001 moles of ions. Given that the self-ionization is a rare event compared to the total number of water molecules, this concentration is extremely low, explaining why the correct answer is d) 0.0000001 moles. The ionization is balanced in water, with equal amounts of hydrogen and hydroxide ions produced, maintaining the neutral nature of pure water.