Question

In a 1.0× 10–6 M solution of HCl(aq) at 25 °C, identify the relative molar amounts of these species from Most to Least: H2O, HCl, Cl-, OH-, H3O+

Answer 1

Answer:

[H₂O] > [H₃O⁺] ≳ [Cl⁻] > [OH⁻] > [HCl]

Explanation:

HCl + H₂O ⟶ H₃O⁺ + Cl⁻

HCl dissociates completely in solution.

[HCl] = 0 mol·L⁻¹

[H₃O⁺] = 1.0 × 10⁻⁶ mol·L⁻¹

[Cl⁻] = 1.0 × 10⁻⁶ mol·L⁻¹

[H₃O⁺] is slightly greater than [Cl⁻] because the autoionization of water also creates some hydronium ion.

1 L of water has a mass of 1000 g.

$\text{Moles of H _{2} O} = \text{1000 g} \times \frac{\text{1 mol}}{\text{18.02 g}} = \text{55.49 mol}$

∴ [H₂O] = 55.49 mol·L⁻¹

$[\text{OH}^{-}] = \frac{K_{w} }{\text{[H}_{3}\text{O}^{+}]} = \frac{1.00 \times10^{-14} }{ 1.0 \times 10^{-6}}= 1.0 \times 10^{-8} \text{ mol/L}$

∴ [H₂O] > [H₃O⁺] ≳ [Cl⁻] > [OH⁻] > [HCl]

   55.5       10⁻⁶        10⁻⁶     10⁻⁸        0