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Leno4ka [110]
4 years ago
7

When you exercise, the burning sensation that sometimes occurs in your muscles represents the buildup of lactic acid (HC3H5O3).

In a .20M aqueous solution, lactic acid is 2.6% dissociated. What is the value of Ka for this acid?
A.) 4.3x10^-6
B.) 8.3x10^-5
C.) 1.4x10^-4
D.) 5.2x10^-3
Chemistry
1 answer:
xenn [34]4 years ago
5 0

Answer:

The Ka for this acid is 1.4 * 10^-4 (option C)

Explanation:

Step 1: Data given

In a 0.20M aqueous solution, lactic acid is 2.6% dissociated.

Step 2: The equation for the dissociation of HAc is:

HAc ⇌ H+ + Ac¯

The Ka expression is:

Ka = ([H+] [Ac¯]) / [HAc]

1) [H+] using the concentration and the percent dissociation:

(0.026) (0.2) = 0.0052 M

2) Calculate [Ac-] and [H+]

[Ac¯] = [H+] = x = 0.0052 M

3) Calculate [HAc]

[HAc] = 0.20M - x ( since x << 0.20, we can assume [HAc]  = 0.20 M

4) Calculate Ka

Ka= [( 0.0052) ( 0.0052)] / 0.20

Ka = 0.0001352 = 1.4 * 10^-4

The Ka for this acid is 1.4 * 10^-4

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Rule of Thirteen:

First divide the parent peak value by 13 as,

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So, the rough formula we have is,

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Now, add one Nitrogen atom to above formula and subtract one Carbon and 2 Hydrogen atoms as these numbers are equal to atomic mass of Nitrogen atom as,

                C₁₂H₁₉   -------N-------->    C₁₁H₁₇N

Also, as shown in ¹³C-NMR there is one peak around 180 ppm and the peak at 1661 cm⁻¹ in IR spectrum is characteristic to carbonyl group hence, we will add one oxygen atom to the chemical formula accordingly. i.e.

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The absence of IR peak above 3200 cm⁻³ also confirms that the amine is tertiary in nature and there is no hydrogen attached to the nitrogen atom.

It can be observed that the peaks in upfield are duplicating. This can be due to the presence of rotamers of said compound.

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