Hydroxide Ion Concentration Of Strong Acids Chemistry Tutorial

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Calculating the Hydroxide Ion Concentration of Strong Monoprotic Acids

Before the acid is added to water, water molecules are in equilibrium with hydrogen ions and hydroxide ions:

water		hydrogenions	+	hydroxide ions
H2O		H+(aq)	+	OH-(aq)

Only a very small number of water molecules dissociate into H+(aq) and OH-(aq). At 25°C, [H+(aq)] = [OH-(aq)] ≈ 10-7 mol L-1 (a very low concentration) and Kw = [H+(aq)][OH-(aq)] = 10-7 × 10-7 = 10-14

When a strong monoprotic acid is added to water, the acid dissociates completely to form H+(aq) and an aqueous anion:

acid	→	hydrogenions	+	anions
HA	→	H+(aq)	+	A-(aq)

Adding the acid to the water disturbs the water dissociation equilibrium: H2O H+(aq) + OH-(aq) By Le Chatelier's Principle, adding more H+(aq) to the water will shift the equilibrium position to the left. The water dissociation equilibrium system responds to the addition of more H+(aq) by reacting some of the H+(aq) with some of the OH-(aq) in order to re-establish equilibrium. So, increasing the concentration of H+(aq) in the water, reduces the concentration of OH-(aq), but, the water dissociation constant does not change4, Kw is still 10-14. So, Kw = [H+(aq)][OH-(aq)] = 10-14

At 25°C	[H+(aq)]mol L-1	[OH-(aq)]mol L-1	Kw
pure water(before acid added)	10-7	10-7	10-14
aqueous solution(after acid added)	> 10-7	< 10-7	10-14

We can use the value of Kw and [H+(aq)] to calculate [OH-(aq)] at a given temperature: At 25°C: Kw = [H+(aq)][OH-(aq)] = 10-14 By rearranging this equation (formula) we can determine the concentration of hydroxide ions in the aqueous solution: [OH-(aq)] = 10-14 ÷ [H+(aq)] Both [H+(aq)] and [OH-(aq)] must be in units of mol L-1 (mol/L or M)

If we know the pH of the acidic solution, we know the concentration of hydrogen ions in the solution because pH = -log10[H+(aq)] By reaarranging this equation (formula) we can find the concentration of hydrogen ions in mol L-1: [H+(aq)] = 10-pH This value of [H+(aq)] can then be used to calculate [OH-(aq)] in mol L-1 using the appropriate value for Kw.

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