An unknown salt is either NaCN, NaC2 H3 O2, NaF, NaCl, or NaOCl . When 0.100 mole of the salt is

An unknown salt is either NaCN, NaC2 H3 O2, NaF, NaCl, or...

Key Concepts

Salt Hydrolysis

Salt hydrolysis refers to the reaction of an ion derived from a weak acid or weak base with water, which can change the pH of the solution. In the context of acid–base equilibria, when a salt is dissolved in water, its anion or cation may react with water to produce OH? or H?, respectively, thereby generating a basic or acidic solution.

Conjugate Acid?Base Pairs

In acid–base chemistry, every weak acid has a conjugate base and every weak base has a conjugate acid. When salts are formed from these species, the conjugate base of a weak acid (or the conjugate acid of a weak base) can undergo hydrolysis. Understanding which ion is a conjugate of a weak acid or base allows prediction of the resulting pH of the salt solution.

Acid and Base Dissociation Constants (Ka and Kb)

The acid dissociation constant (Ka) and the base ionization constant (Kb) quantify the strength of acids and bases, respectively. For an ion undergoing hydrolysis, Kb or Ka is used to determine how much the ion will react with water, which in turn influences the pH. These constants are essential for calculating equilibrium concentrations in aqueous solutions.

Relationship between Ka, Kb, and Kw

The equilibrium constants for conjugate acid–base pairs are interconnected through the water ionization constant (Kw) by the relation Ka × Kb = Kw. This relationship is crucial because it allows calculation of the basicity (Kb) of an ion if the acidity (Ka) of its conjugate acid is known, aiding in predicting the pH of salt solutions.

pH Calculation for Weak Base Solutions

When calculating the pH of a solution formed from the salt of a weak acid, the hydrolysis of the anion is considered. The basic pH is determined by setting up an equilibrium expression using the known Kb value and approximating hydroxide ion concentrations. This process involves solving the equilibrium equation and subsequently converting hydroxide ion concentration to pH, which is a fundamental technique in solution chemistry.

Transcript

00:01 So in this question, we're told that whenever one mole of one of these five salts is dissolved in a liter of water, the ph for the resulting solution is 8 .07.

00:12 So let's take a look at what these salts are.

00:14 They all have the same cation, sodium.

00:18 And sodium is a neutral ion, and so this will play no effect on the ph.

00:23 And so really what we need to do is we need to figure out which of these anions, cn minus, acetate, fluoride, chloride, or ocl minus is going to result in a ph of 8 .07 at that certain concentration.

00:40 So what we want to do is we want to figure out, use the concentration and this ph in order to figure out the kb of whatever salt this is.

00:49 And then we can go from there to figure out what the identity is.

00:52 And so we're told that we have 0 .1 mole, which is dissolved in 1 liter.

00:59 And so vary directly.

01:01 This is 0 .1 molar of whatever salt we have.

01:05 And so we're going to use this ph now and our concentration in order to figure out what the kb is.

01:12 And so kb, whenever you do an ice table for a base reacting with water to be in equilibrium with its conjugate acid and hydroxide, is always going to end up being the concentration of hydroxide times the concentration of the conjugate acid.

01:32 Over the base initially, the concentration of the base.

01:36 And so after a nice table, you'll always find that this is equal to x squared over the initial concentration, which in this case is 0 .1 minus x, where x is equals to the concentration of oh h minus and the conjugate acid...

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