Calculate [OH^-], percent ionization, and pH for (a) 0.25 M aqueous ammonia, (b) 0.25 M methylam

Calculate [OH^-], percent ionization, and pH for (a) 0.25 M...

Key Concepts

pH and pOH Relationship

The pH of a solution, which indicates its acidity or basicity, is interconnected with the hydroxide ion concentration via pOH. pOH is determined from the [OH?] by using the formula pOH = -log[OH?], and then the pH is calculated by subtracting pOH from 14.

Percent Ionization

Percent ionization is a measure of the fraction of a weak base that ionizes in solution, expressed as a percentage of the initial concentration. It provides insight into the efficiency of ionization and the behavior of the base in solution.

Base Ionization Constant (Kb)

The equilibrium constant, Kb, quantifies the strength of a weak base by relating the concentrations of the products (the conjugate acid and hydroxide ions) to the concentration of the un-ionized base. It is central to predicting how much of the base will ionize in water.

Equilibrium Reactions of Weak Bases

Weak bases such as ammonia and methylamine react with water in an equilibrium process to form their conjugate acid and hydroxide ions. Understanding this reaction is key to determining the extent of ionization and the resulting concentration of hydroxide ions in solution.

Hydroxide Ion Concentration Calculation

Determining the concentration of hydroxide ions ([OH?]) involves setting up and solving the equilibrium expression for the weak base reaction. Often, approximations are used when the ionization is minimal compared to the initial concentration, simplifying the calculation.

Transcript

00:01 To calculate concentration of oh minus percent ionization and the ph for .25m ammonia solution and then 0 .25 molar methylamine solution.

00:16 Let's start with the ammonia solution.

00:20 Okay, let's start with the equation nh3 plus water, nh4 plus plus oh minus.

00:41 Let's make the ice table here.

00:56 Okay, initial concentration is 0 .2500.

01:04 Change concentration is minus x plus x.

01:10 Plus x and equilibrium concentration 0 .25 minus x x x.

01:19 Let's have the expression for kb.

01:25 That is the concentration of nh4 plus times concentration of o, which minus, divided by the concentration of ammonia.

01:36 Now, kb for ammonia is 1 .8 times 10 to the power negative 5.

01:48 And here, nh4 plus and o is minus concentration x, so it will be x squared, and for concentration of ammonia in equilibrium, 0 .25 minus x.

02:03 Now, here we are making the simplifying assumption that x is much, much smaller than 0 .25, so we are ignoring this x.

02:16 Therefore, x squared is 1 .8 times 0 .25 times 10 to the power minus 5.

02:25 Therefore, x will be square root of 1 .8 times 0 .25 times 10 to the power minus 5.

02:33 10 to the power negative 5.

02:36 And the value of x is 0 .00212, that is 2 .12 times 10 to the power negative 3.

02:49 And x is concentration of, therefore, concentration of oh minus ion is 2 .12 times 10 to the power negative 3m.

03:03 Therefore, concentration of hydrogen ion, we have to calculate.

03:09 Now we know concentration of h plus ion and oh minus ion is kw.

03:16 Kw is an product constant of water.

03:20 Therefore, concentration of hydrogen ion will be kw, which is 1 into 10 to the power negative 14, divided by the concentration of which minus ion which is 2 .12 times 10 to the power negative 3 then the concentration of h plus is 4 .72 times 10 to the power negative 12 that's the hydrogen plus ion concentration therefore ph which is negative negative log of hydrogen ion concentration and the value is 11 .3.

04:11 Now we'll calculate the percent ionization.

04:15 Percent ionization is concentration of oh, which is 2 .12 times 10 to the power negative 3 divided by the initial concentration 0 .25.

04:35 Times 100 and the value of percent ionization is 0 .85 percent.

04:44 And our assumption, simplifying assumption is valid because percent ionization is much, much less than 5 percent, and much less than the original initial concentration of 0 .25m...

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