Calculate the mass of benzoic acid, C6 H5 COOH, which must be dissolved to prepare 1.00 L of a

Calculate the mass of benzoic acid, C6 H5 COOH, which must...

Key Concepts

pH and Hydrogen Ion Concentration

The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration (pH = -log[H+]). This relationship is fundamental because it allows us to directly calculate the concentration of H+ ions from a known pH, which is crucial when determining how much of an acidic substance is needed to reach a desired pH level in solution.

Acid Dissociation Equilibrium

Weak acids do not completely ionize in water; instead, they establish an equilibrium between the undissociated acid (HA) and its dissociated ions (H+ and A-). The equilibrium is described by the expression Ka = [H+][A-]/[HA]. Understanding this equilibrium is key when preparing solutions with a specific pH, as it helps in determining how much of the weak acid will dissociate versus remain undissociated.

Acid Dissociation Constant (Ka and pKa)

The acid dissociation constant (Ka) measures the strength of an acid in solution while pKa, its negative logarithm, provides a more convenient scale. These constants are critical for assessing the extent of acid ionization in water and for solving equilibrium problems involving weak acids. They allow us to relate the initial concentration of the acid to the concentration of hydrogen ions in a solution at equilibrium.

Stoichiometry and Mole-to-Mass Conversion

Once the required molar concentration of the acid is determined from the equilibrium calculations, stoichiometric principles are used to convert this concentration into an actual mass. This conversion involves calculating the moles of acid needed for a given volume and then using the molar mass of the acid to find the corresponding mass. This step is essential in practical laboratory settings where precise amounts of a compound must be weighed out for solution preparation.

Transcript

00:01 In answering this question, we have to recall that the ph is equal to negative log concentration of the h3 or positive ions.

00:09 Now, what this means is the concentration of the h3 or positive ions is going to put 10 to the power negative ph, which in this case is equal to 3 .5, which is equal to 3 .2 multiplied by 10 to the power negative 4 in units of molarity.

00:25 So what this is telling us right here, this is the concentration of h3 or positive ions that must be supplied by the benzoyic acid in order to produce a solution that is equal to 3 .5, whose ph is equal to 3 .5.

00:42 So what we want to do here is to determine the volume of the benzoy acid rather the mass of the benzoy acid that when dissolved in solution, it will produce a concentration of h.

00:55 3 or positive ions of 3 .2 by 10 to the power negative 4 which corresponds to a ph of 3.

01:02 Now looking at this what we are doing here is we have the benzoic acid that is going to dissolve in solution to produce the benzoyac acetate or rather let's just write this as c6 h5 c.

01:19 That dissolves in solution to produce c6 this will be in h2 or producing c6 h5 c0 negative plus h3o positive.

01:32 So looking at this, we've got an initial concentration that we do not know.

01:37 And because before the actual ionization takes place, we won't be having any of these ions in the solution.

01:44 So looking at the change, we are looking at a decrease by 3 .2 by 10 to the power negative 4.

01:50 We are looking at an increase by 3 .2 by 10 to the power negative 4.

01:54 We're looking at an increase of 3 .2 by 10 to the power negative 4 because what we are looking for at equilibrium is this concentration of 3 .2 by 10 to the power negative 4...

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