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A sodium hydroxide solution was titrated against citric acid (C6H8O7) which is triprotic - HSC - SSCE Chemistry - Question 26 - 2015 - Paper 1
Question 26
A sodium hydroxide solution was titrated against citric acid (C6H8O7) which is triprotic. (a) Draw the structural formula of citric acid (2–hydroxypropane–1,2,3–tri... show full transcript
Worked Solution & Example Answer:A sodium hydroxide solution was titrated against citric acid (C6H8O7) which is triprotic - HSC - SSCE Chemistry - Question 26 - 2015 - Paper 1
Step 1
Draw the structural formula of citric acid (2–hydroxypropane–1,2,3–tricarboxylic acid)
Answer
The structural formula of citric acid includes three carboxylic acid groups (-COOH) and one hydroxyl group (-OH) attached to a central carbon atom. It can be depicted as follows:
OH
|
HOOC-C-COOH
|
C
|
HOOC-
This representation illustrates the connectivity of atoms in the citric acid molecule.
Step 2
How could a computer-based technology be used to identify the equivalence point?
Answer
A digital pH probe could be utilized to collect data for plotting a graph of pH versus the volume of sodium hydroxide. The equivalence point can be accurately determined from the inflection point on the pH curve, where there is a sharp increase in pH indicating the complete neutralization of citric acid.
Step 3
Calculate the concentration of the sodium hydroxide solution.
Answer
To find the concentration of the sodium hydroxide solution, we first determine the number of moles of citric acid used:
- Volume of citric acid = 25.0 mL = 0.0250 L
- Concentration of citric acid = 0.100 mol L⁻¹
- Moles of citric acid = Volume x Concentration = 0.0250 L x 0.100 mol L⁻¹ = 0.00250 moles
The neutralization reaction between citric acid (C₆H₈O₇) and sodium hydroxide (NaOH) is as follows:
This shows that 1 mole of citric acid reacts with 3 moles of NaOH. Therefore, moles of NaOH used = 3 x moles of citric acid = 3 x 0.00250 moles = 0.00750 moles.
Next, we calculate the concentration of NaOH:
- Volume of NaOH used = 41.50 mL = 0.04150 L
- Concentration (c) of NaOH can be calculated using the formula:
c = \frac{0.00750 \text{ moles}}{0.04150 \text{ L}} = 0.1807 \text{ mol L}^{-1}$$ Rounding to three significant figures, the concentration of the sodium hydroxide solution is 0.181 mol L⁻¹.