How to Determine if a Species is Charged, Neutral, or Unknown
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How to Determine if a Species is Charged, Neutral, or Unknown
Can someone please explain how we know if a weak acid or weak base is charged, neutral, or unknown if the pH is less than the pKa? What is the relationship between pH and pKa?
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Re: How to Determine if a Species is Charged, Neutral, or Unknown
pH is the measure of how basic or acidic a solution is, whereas pKa and pKb represent how strong a specific acid or base is, depending on its equilibrium concentration of H3O+ or OH-.
For a weak acid HA, the reaction with water is as follows: HA (aq) + H2O (l) --> A- (aq) + H3O+ (aq).
If pKa for an acid is less than the pH of a solution, the acid will exist in the uncharged HA protonated form. If pKa is greater than pH, that means the equilibrium acidity/deprotonation of the acid has not yet been "achieved," so the acid will deprotonate and exist in the charged form, or A- form.
For a weak base B, the reaction with water is as follows: B (aq) + H2O (l) --> BH+ (aq) + OH- (aq).
If pKb for a base is less than the pH of a solution, the base will exist in the charged BH+ form. This is because the solution is not at the equilibrium basicness that the acid typically exists at, so the base will accept more protons to form BH+. If pKb is greater than pH, the base will exist in the uncharged B form.
For a weak acid HA, the reaction with water is as follows: HA (aq) + H2O (l) --> A- (aq) + H3O+ (aq).
If pKa for an acid is less than the pH of a solution, the acid will exist in the uncharged HA protonated form. If pKa is greater than pH, that means the equilibrium acidity/deprotonation of the acid has not yet been "achieved," so the acid will deprotonate and exist in the charged form, or A- form.
For a weak base B, the reaction with water is as follows: B (aq) + H2O (l) --> BH+ (aq) + OH- (aq).
If pKb for a base is less than the pH of a solution, the base will exist in the charged BH+ form. This is because the solution is not at the equilibrium basicness that the acid typically exists at, so the base will accept more protons to form BH+. If pKb is greater than pH, the base will exist in the uncharged B form.
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Re: How to Determine if a Species is Charged, Neutral, or Unknown
Looking at the relationship between pH and pKa: the higher the pKa, the weaker the acid (higher the pH); the lower the pKa, the stronger the acid (higher the pH). This is because the Ka and Kb have an inverse relationship, therefore meaning that the greater the Ka the more acidic the reaction is at equilibrium (pKa is the -log of Ka so a greater Ka means a lower pKa).
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