Question

A 0.210 -g sample of an acid (molar mass \(=192 \mathrm{g} / \mathrm{mol}\) ) is titrated with 30.5 \(\mathrm{mL}\) of 0.108\(M \mathrm{NaOH}\) to a phenolphthalein end point. Is the acid monoprotic, diprotic, or triprotic?

Short answer

The acid is likely triprotic, as the calculated ratio of moles of NaOH to moles of the acid is approximately 3, indicating that it has three acidic protons.

Step-by-step solution

Calculate moles of the acid

To find the moles of the acid, we can use the formula: moles = mass / molar mass. We are given the mass (0.210 g) and molar mass (192 g/mol) of the acid. Let's calculate the moles of the acid: Moles of the acid = \(\frac{0.210 \,\text{g}}{192\, \frac{\text{g}}{\text{mol}}}\) Moles of the acid = \(1.094 \times 10^{-3}\) mol

Calculate moles of NaOH

To find the moles of NaOH, we can use the formula: moles = volume × concentration. We are given the volume (30.5 mL = 0.0305 L) of NaOH and its concentration (0.108 M). Let's calculate the moles of NaOH: Moles of NaOH = (0.0305 L) × (0.108 mol/L) Moles of NaOH = 0.00329 mol

Calculate the ratio of moles of NaOH to molesh

Now, we will calculate the ratio of moles of NaOH to moles of the acid: Ratio = \(\frac{\text{moles of NaOH}}{\text{moles of the acid}}\) Ratio = \(\frac{0.00329\, \text{mol}}{1.094 \times 10^{-3}\, \text{mol}}\) Ratio = 3.005

Determine the number of acidic protons

The ratio of moles of NaOH to moles of the acid is approximately 3. Since the ratio is close to 3, we can conclude that the acid is likely triprotic, meaning it has three acidic protons.

Key concepts

Acid-Base Reaction

Acid-base reactions are a type of chemical interaction where an acid and a base neutralize each other. In the context of titration, an acid reacts with a base to produce water and a salt. This process is crucial for determining the concentration of an unknown solution. The titration you performed involves an acid being gradually reacted with a known concentration of a base under controlled conditions.
This helps identify characteristics of the acid, such as its acidity, including whether it is monoprotic (one proton), diprotic (two protons), or triprotic (three protons). Understanding this helps in calculating how much base is required to completely neutralize the acid, which directly ties into the determination of the acid's properties.

Equivalence Point

The equivalence point in a titration marks the stage at which the amount of acid equals the amount of base added, indicating that all the acid has reacted with the base. This is a vital concept because it signifies complete neutralization. In our titration exercise, we aim to reach this point to determine the nature of the acid.
A vivid sign of reaching the equivalence point is the change of the solution color if a pH indicator like phenolphthalein is used. The choice of a suitable indicator is important, as it must change color close to the pH level at the equivalence point for accurate measurements.

• For a monoprotic acid, the equivalence point occurs at a 1:1 acid to base mole ratio.
• For a diprotic acid, it typically occurs at a 1:2 ratio.
• For a triprotic acid, a 1:3 ratio, as encountered in the solution, suggests the equivalence point.

Proper calculation of the equivalence point is crucial for deducing the type of acid present in your solution.

Molarity

Molarity (M) is a measure of concentration that tells us how many moles of a solute are in one liter of solution. It's a key concept in titration, as it allows us to determine the number of moles of a reactant or product in a given volume.
In the exercise, the molarity of the sodium hydroxide (NaOH) solution was used to find out how much of it was needed to neutralize the acid. This is done through the equation: \[ \text{Molarity} = \frac{\text{moles of solute}}{\text{liters of solution}} \]For example, the NaOH in the titration has a molarity of 0.108 M, indicating its concentration. It played a decisive role in reaching the equivalence point by telling us how many moles of the base were present in the solution used for titration.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions and is fundamental in titrometric analysis. This involves using balanced chemical equations to relate moles of reactants to moles of products, crucial for quantifying titration results.
By calculating the moles of each substance involved in your experiment, you can establish the stoichiometric relationship between them. In the titration exercise, the ratio of NaOH to the acid was determined to be approximately 3:1. This ideally fits a triprotic acid, confirming the stoichiometry of the reaction. Understanding stoichiometry is vital for performing proper titrations, as it ensures that the calculations accurately reflect the observed chemical behavior.
Some helpful points:

• Utilizing the molar mass and volume to determine moles is critical.
• Accurate measurements lead to identifying the stoichiometric coefficients.
• The coefficients show how many molecules or moles of each substance react or are produced.

The stoichiometry was evaluated effectively in this exercise, reinforcing the concept that the acid involved was indeed triprotic.