Calculate the concentration of all solute species in each of...

Key Concepts

Assumptions and Approximations

In many acid-base equilibrium calculations, especially with weak acids or bases, certain assumptions can be made such as neglecting the ionization of water and assuming that the change in initial concentration is negligible. These approximations simplify the mathematical treatment of the problem and are valid when the degree of ionization is very small relative to the initial concentration of the acid or base.

Equilibrium Expressions

Setting up equilibrium expressions is necessary to relate the concentrations of the reactants and products at equilibrium for both weak acids and weak bases. These expressions utilize the known dissociation constants (Ka or Kb) and are typically derived from the law of mass action. The correct formulation of these expressions is key to solving for unknown concentrations in acid-base equilibria.

ICE Table Method

The ICE Table (Initial, Change, Equilibrium) is a systematic approach used to organize and solve equilibrium problems. It helps in clearly identifying the initial concentrations, the changes that occur when equilibrium is established, and the final equilibrium concentrations. This method is especially useful when making approximations about small changes in concentration for weak acids and bases.

Weak Acids

This concept involves understanding that weak acids do not fully dissociate in aqueous solution. Instead, they reach an equilibrium between the undissociated acid and the ions produced upon partial ionization. The acid dissociation constant (Ka) is used to quantify the strength of the weak acid and assess the position of the equilibrium. This fundamental concept is essential when calculating the concentrations of all species in solution.

Weak Bases

Weak bases are those that only partially accept protons from water, establishing a chemical equilibrium between the unprotonated base and its conjugate acid along with hydroxide ions. Their base dissociation constant (Kb) determines the extent of their reaction in solution. Understanding weak base ionization is crucial for computing various ion concentrations when the base is dissolved in an aqueous environment.

Transcript

00:02 Let's calculate the concentration of all solute species in each of the following acids or bases, and we'll have to apply the 5 % rule to see if the assumptions we make are valid.

00:14 For a, we have a weak acid of 0 .0092 molar hcl.

00:22 We were told that as a weak acid.

00:24 We can look up its k -a value, which is 3 .5 times 10 to the negative 8.

00:34 It's calculate the concentration of all species, first by starting to write bronzed lower equilibrium for weak acid.

00:52 Initial malarity, complete a rice table.

01:04 Now assumption here, we're going to assume that 0 .0092 minus x is approximately equal to 0 .0092.

01:14 And our k -a expression, ka is 3 .5 times 10 of the negative 8.

01:36 Solving for x i get 1 .8 times 10 to the negative 5 let's verify our assumption so assumption here 5 % of 0 .0092 is equal to 4 .6 times 10 to the negative 4 so x which is 1 .8 times 10 to the negative 5 is less than 4 .6 times 10 to the negative 4.

02:32 Therefore, since it is less, according to the 5 % rule, the assumption is valid.

02:42 And now we can report the concentration of all species, the concentration of h -3 -0 plus, it's equal to the concentration of clo -minus.

02:50 And where i table, this was x, which is equal to 1 .8 times 10 to the negative 5 molar.

02:55 And the concentration of hclo is equal to 0 .0092 molar as our assumption is valid.

03:03 For b, we have 0 .0784 molar, c6, h5, and h2, and we're told that it is a weak base for the weak base.

03:25 We'll look up the kb value, which is equal to 4 .6.

03:31 Times 10 to the minus 10.

03:33 Let's calculate the concentration of all species.

03:36 Let's write the equation for the weak base.

03:53 And the similarity is 0 .0784.

03:57 Complete a rice table here.

04:06 Kb expression.

04:23 We know the kb values.

04:24 We're going to assume here.

04:26 I didn't write the assumption here.

04:29 But we will, i'll squeeze it in here.

04:32 We're going to assume that 0 .0784 minus x is approximately equal to 0 .0784.

04:42 We'll include that in our expression here.

04:47 The kb we found out was 4 .6 times 10 to the negative 10 from the appendix.

04:52 It's equal to the x, x.

04:56 And our assumption here is 0 .0784.

05:01 Solving for x here, we find that x is equal to 6 .0 times 10 to the negative 6.

05:09 Let's word this above here.

05:14 Let's look at our assumption here.

05:21 5 % of 0 .0784 is equal to 3 .9 times 10 to the negative 3 .3.

05:31 Therefore, x, which is 6 .0 times 10 to the negative 6, is less than 3 .9 times 10 to the negative 3.

05:41 Therefore the assumption is valid.

05:51 Concentration of c6h5nh3 plus is equal to the concentration of oh -1 -8 -1.

06:00 And there are ice table that's they're both equal to x.

06:02 So these are both equal to 6 .0 times 10 to the negative 6 molar.

06:06 And the molarity of the c6h5 and h2 is equal to 0 .0784 molar.

06:16 There would be the concentration of all species.

06:25 For c, we have weak acid .08810 molar hcn.

06:37 We can look up the ca value for hcn, which is equal to 4 .0 times 10 to the negative 10.

06:49 Bronset -lauri equation for hcn.

07:03 Initial malarity, 0810 minus x, x, x, and x...

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