You weigh a sample of a monoprotic unknown acid and dissolve it in 50.00 mL of distilled water. Exactly half of this solution is titrated with Sodium Hydroxide to the phenolphthalein end point.
The pH of the other half of the original solution is measured with a pH meter. The "neutralized" solution is added to the "original" solution and the pH of this combined "final" solution is also measured.
The following are the measured values:
Mass of unknown acid 1.6214 g
Volume of NaOH used in titration 15.88 mL
Concentration of the NaOH used 0.1849 M
pH of the original acid solution 2.64
pH of the final acid solution 4.45
CALCULATE the following
(a) Molecular Weight of Acid used in titration ___________________
(b) Molarity of UNKNOWN Acid solution from titration ___________________
(c) Ka of UNKNOWN Acid ___________________
(d) Concentration of undissociated Acid from pH measurements ___________________
(e) Total concentration of UNKNOWN acid from pH measurements ___________________
To calculate the required values, we will need to use some key equations and principles of acid-base titration and pH measurements. Let's go through each calculation step by step:
(a) Molecular weight of Acid used in titration:
To calculate the molecular weight, we need to determine the number of moles of the acid used in the titration. We can find this by dividing the mass of the acid by its molar mass.
Given:
Mass of unknown acid = 1.6214 g
To calculate the molar mass, more information about the acid is required. If you have the chemical formula or any additional information, please provide it so that we can move forward with the calculation.
(b) Molarity of UNKNOWN Acid solution from titration:
To determine the molarity of the unknown acid, we can use the balanced chemical equation between the acid and sodium hydroxide. The mole-to-mole ratio will allow us to calculate the molarity.
The balanced equation between the acid (HA) and sodium hydroxide (NaOH) is:
HA + NaOH -> NaA + H2O
Given:
Volume of NaOH used in titration = 15.88 mL
Concentration of NaOH used = 0.1849 M
First, convert the volume of NaOH used to liters (L):
15.88 mL = 15.88 * 10^(-3) L
Next, use the mole-to-mole ratio from the equation:
1 mole of HA reacts with 1 mole of NaOH
Moles of NaOH = Volume of NaOH (in L) * Concentration of NaOH
Moles of NaOH = 15.88 * 10^(-3) L * 0.1849 M
Finally, since the reaction has a 1:1 mole ratio, the moles of NaOH will be equal to the moles of HA in the original solution.
(c) Ka of UNKNOWN Acid:
To calculate the Ka (acid dissociation constant) of the unknown acid, we need the concentration of the unknown acid solution and the concentration of the undissociated acid.
To start, we can calculate the concentration of the unknown acid solution based on the moles of the acid and its volume.
Given:
Volume of the original solution = 50.00 mL
Moles of the acid (from part (b)) = [calculate above]
Convert the volume of the original solution to liters (L):
50.00 mL = 50.00 * 10^(-3) L
Concentration of the unknown acid solution = Moles of the acid / Volume of the original solution
The undissociated acid concentration can be determined from the pH measurement. Since the pH of the original acid solution is given, we can use it to calculate the concentration of undissociated acid.
(d) Concentration of undissociated Acid from pH measurements:
pH is a measure of the concentration of hydrogen ions (H+) in a solution. By using the pH of the original acid solution, we can calculate the concentration of H+ ions and thus the concentration of undissociated acid.
Given:
pH of the original acid solution = 2.64
Since the pH is given, we can calculate [H+] using the formula:
[H+] = 10^(-pH)
The concentration of the undissociated acid will be equal to [H+] in the solution.
(e) Total concentration of UNKNOWN acid from pH measurements:
The total concentration of the unknown acid in the final solution can be calculated by summing the concentrations of undissociated acid and the dissociated acid.
Given:
pH of the final acid solution = 4.45
Using the same method as in part (d), calculate the concentration of undissociated acid from the pH of the final solution.
The total concentration of the unknown acid will be the sum of the concentrations of undissociated acid and dissociated acid.
Please provide the additional information needed for the calculations, such as the chemical formula or any other relevant data, so that we can proceed with the calculations.
To solve the given problems, we can follow these steps:
(a) Molecular Weight of Acid used in titration:
Step 1: Calculate the number of moles of NaOH used in the titration.
Moles of NaOH = Molarity × Volume in liters
Volume of NaOH used = 15.88 mL = 15.88/1000 L
Moles of NaOH = 0.1849 M × (15.88/1000) L
Step 2: Since a 1:1 reaction occurs between the unknown acid and NaOH, the number of moles of acid is equal to the number of moles of NaOH.
Moles of acid = Moles of NaOH
Step 3: Calculate the Molecular Weight of the unknown acid.
Molecular Weight = Mass/Moles
Molecular Weight = 1.6214 g / Moles of acid
(b) Molarity of UNKNOWN Acid solution from titration:
Step 1: Calculate the number of moles of NaOH used in the titration (same as in part a).
Step 2: Calculate the volume of the unknown acid solution used in the titration.
Volume of unknown acid solution = Volume of NaOH used / 2
Volume of unknown acid solution = 15.88 mL / 2
Step 3: Calculate the concentration of the unknown acid solution.
Molarity = Moles / Volume in liters
Molarity = Moles of acid / (Volume of unknown acid solution / 1000)
(c) Ka of UNKNOWN Acid:
Step 1: Calculate the concentration of the acid solution using the pH measurement.
Concentration = 10^(-pH)
Concentration = 10^(-2.64)
Step 2: Calculate the concentration of the undissociated acid.
Concentration of undissociated acid = Concentration of acid solution / (1 + 10^(-pKa))
Step 3: Calculate the Ka value.
Ka = (concentration of undissociated acid)^2 / (concentration of acid solution - concentration of undissociated acid)
(d) Concentration of undissociated Acid from pH measurements:
Already calculated in part (c).
(e) Total concentration of UNKNOWN acid from pH measurements:
Step 1: Calculate the concentration of the acid solution using the pH measurement (same as in part (c)).
Step 2: Calculate the concentration of the dissociated acid.
Concentration of dissociated acid = Concentration of acid solution - concentration of undissociated acid
Step 3: Calculate the total concentration.
Total concentration = concentration of undissociated acid + concentration of dissociated acid
Please plug in the known values from the given data to calculate the missing values in each step.