El ácido láctico, CH3 CH(OH) COOH, debe su nombre al hecho de que está presente en la leche agri

step 1 So I'll just get started. Lactic acid, which is, I better write this down, the H3, I'm going to

El ácido láctico, CH3 CH(OH) COOH, debe su nombre al hecho...

El ácido láctico, $\mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{COOH}$, debe su nombre al hecho de que está presente en la leche agria como producto de la acción bacteriana. Asimismo, es la causa del adolorimiento de los músculos después de un ejercicio vigoroso. (a) El $\mathrm{pK}$ del ácido láctico es de 3.85. Compárelo con el valor correspondiente al ácido propiónico $\left(\mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COOH}, \mathrm{pK}_{\Omega}=4.89\right)$ y explique la diferencia. (b) Calcule la concentración de ion lactato en una disolución $0.050 \mathrm{M}$ de ácido láctico. (c) Cuando se mezcla una disolución de lactato de sodio, $\left(\mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{COO}\right) \mathrm{Na}$, con una disolución acuosa de cobre(II), es posible obtener una sal sólida de lactato de cobre(II) en forma de hidrato azul verdoso: $\left(\mathrm{CH}_3 \mathrm{CH}(\mathrm{OH}) \mathrm{COO}\right)_2 \mathrm{Cu} \cdot x \mathrm{H}_2 \mathrm{O}$. El análisis elemental del sólido indica que éste contiene $22.9 \%$ de Cu y $26.0 \%$ de C en masa. ¿Cuál es el valor de $x$ en la fórmula del hidrato? (d) La constante de disociación ácida del ion $\mathrm{Cu}^{2+}(a c)$ es de $1.0 \times 10^{-8}$. Con base en este valor y en la constante de disociación ácida del ácido láctico, prediga si una disolución de lactato de cobre(II) será ácida, básica o neutra. Explique su respuesta.

Key Concepts

Coordination Compounds and Hydration

Coordination compounds consist of a central metal ion bonded to surrounding ligands, and they can include water molecules either coordinated directly to the metal or in the crystal lattice as water of hydration. Understanding the structure and composition of such compounds is essential, particularly when predicting properties like color, solubility, and reactivity.

Acid Dissociation Constant (pKa)

This concept refers to the quantitative measure of the strength of an acid in solution. The pKa value indicates the tendency of an acid to donate a proton, with a lower pKa signifying a stronger acid. In the context of comparing different carboxylic acids, understanding pKa helps explain differences in acid strength based on molecular structure and substituent effects.

Acid-Base Equilibrium Calculations

This involves the application of equilibrium expressions to determine the concentration of species in solution that are in equilibrium, such as the concentration of a conjugate base from a weak acid. These calculations typically use the acid dissociation constant (Ka) or its logarithmic counterpart (pKa) and may involve the Henderson-Hasselbalch equation to relate pH, pKa, and the ratio between the acid and its conjugate base.

Elemental Analysis in Determining Chemical Formulas

Elemental analysis is a method used to determine the percentage composition of elements within a compound. By comparing the experimentally obtained mass percentages with the theoretical values based on assumed formulas, it is possible to deduce the empirical formula of a compound, including the number of water molecules in a hydrate, which is crucial in the analysis of coordination compounds.

Salt Hydrolysis and pH Prediction

This concept involves the reaction of ions from dissolved salts with water, which can alter the pH of the solution. The hydrolysis of metal cations or the conjugate base of a weak acid can lead to acidic or basic solutions. Knowledge of the acid and base dissociation constants enables the prediction of the resultant pH when salts derived from weak acids or bases are dissolved in water.

Transcript

00:01 So i'll just get started.

00:03 Lactic acid, which is, i better write this down, the h3, i'm going to make my textbook a little bigger, c .h.

00:26 Received its name because it's present in sour milk as a product of bacterial action.

00:33 It's also responsible for soreness in muscles.

00:39 It's sort of interesting factoids.

00:52 For part a, the p .k .a of lactic acid is 3 .85.

01:15 Compare this with the value for propionic acid, 4 .89, and explain the differences.

01:56 Okay, let's go with a lovely red here.

02:03 The structure here is similar, but the lactic acid has the oh group in here.

02:12 So this is what our big difference is right there.

02:21 This is an electronegative substituent withdraws electronid density from the c -o -o -h group.

03:17 That stabilizes the conjugate base, which increases the strength of lactic acid relative to propionic acid.

03:55 And i'm going to leave the acid off.

04:24 I get this raised here again.

04:29 Okay, and then as a little note, a stronger acid equals larger k -a, which is smaller p -k -a.

04:46 Okay, that's part a.

04:49 Let's go to part b.

04:52 There's four parts, so sorry.

05:00 Oops, part b.

05:04 Calculate the lactic acid concentration.

05:07 I'm going to abbreviate lactic acid la.

05:13 Find the lactic acid concentration in a 0 .050 molar solution of lactic acid.

05:29 Okay, this one's easy.

05:35 So we already said that our pca, which was given, was 3 .85, which means that our ka is 10 to the minus 3 .85, which equals 1 .4 times 10 to the minus 4th.

05:59 And i don't feel like doing the whole equation, but, i guess i will.

06:24 These are all aq.

06:37 There's a minus up there, i ran out of room.

06:41 And if we look at our concentrations here, and our k -a expression is, again, i'm going to switch to lactic acid so i don't have to write that whole thing out.

07:15 And oh man, i'm going to go to the lactic acid ion, so i don't have to write the whole thing out.

07:23 I'll color code these quick.

07:24 This is this this this i did that backwards this is my ion this is this this is this there we go sorry about that excuse me so let's go ahead here our ka is pretty small usually this is iffy for me and i'm pretty since i know how to solve for the quadratics, i'm usually pretty good, but i didn't use a quadratic on this.

08:38 So this will equal x squared over 0 .050 minus x, which is very close to x squared over 0 .050.

08:53 Solving for x, we get 2 .6 times 10 to the...

08:59 Actually, i got 2 .646 times 10 to the minus third, which i rounded 2 .6.

09:07 Times 10 to the minus third molarity, and that's my concentration of my ion.

09:22 Let me go back and see what my question was again.

09:31 Make sure i've answered the question.

09:37 Calculate the lactic ion concentration.

09:40 Okay, so we were told to calculate the lactate ion concentration, which we just calculated as 2 .6 times 10 to the minus 3 molar.

09:54 Very good.

09:59 Now c.

10:10 When a solution of sodium lactate is mixed with an aqueous copper 2 solution, so this is just going to be the whole thing with c -o -o -n -a, is mixed with, what did i say, copper -2 solution.

11:05 It is possible to get a solid salt, which is a blue -green copper -to -hotylactate, which is a blue -green hydrate.

11:34 So it will look like this.

11:36 It's not look like this.

11:56 Make sure i've got that down.

11:59 It looks like that's correct.

12:01 Elemental analysis tells us that this substance is 22 .9 % copper.

12:11 26 .0 % carbon by mass.

12:21 Find the value for x.

12:32 Okay, this shouldn't be too hard.

12:37 So we're going to assume 100 gram sample.

12:43 This is an assumption.

12:46 And we're going to calculate the moles of copper in the sample...