How to Determine the Concentration of Hydrochloric Acid by Titration?

You can determine the concentration of hydrochloric acid (HCl) by titrating it with a strong base of known concentration.

What is Titration?

Titration is a quantitative chemical analysis technique used to determine the concentration of an unknown substance (the analyte) by reacting it with a solution of known concentration (the titrant). In this case, the analyte is the HCl solution, and the titrant is a strong base like sodium hydroxide (NaOH).

The principle is based on a neutralization reaction, where the acid and base react until the moles of acid equal the moles of base, a point called the equivalence point.

The Titration Process: Step-by-Step

Determining the concentration of HCl involves carefully reacting a measured volume of the acid with a strong base until the reaction is complete. Here's a breakdown:

Prepare the Setup:
- A known volume of the HCl solution with unknown concentration is accurately measured using a pipette and placed into a conical flask.
- A burette is filled with the strong base solution of known concentration (the standard solution). The initial volume is recorded.
- An appropriate indicator (e.g., phenolphthalein) is added to the HCl in the conical flask. Phenolphthalein is commonly used because it changes color sharply near the neutral point (pH 7), which corresponds to the equivalence point for a strong acid-strong base titration.
Perform the Titration:
- The strong base is slowly added dropwise from the burette into the conical flask containing the HCl and indicator, while continuously swirling the flask.
- As the base is added, it neutralizes the acid. The indicator will show transient color changes where the base hits the solution, but these disappear upon swirling as the reaction occurs.
- The addition continues until a single drop of the base causes a permanent color change throughout the solution (e.g., faint pink for phenolphthalein). This is the endpoint of the titration, which is assumed to be very close to the equivalence point.
Record the Volume:
- The final volume of the base remaining in the burette is recorded.
- The volume of base added is calculated by subtracting the initial volume from the final volume. This is the titration volume.
Calculations:
- Calculate Moles of Base: Using the known concentration (molarity, M) of the strong base and the volume used in the titration, calculate the number of moles of the strong base required to reach the equivalence point.
  - Reference Step 1: "First, calculate the number of moles of strong base required to reach the equivalence point of the titration."
  - Formula: Moles = Concentration (M) × Volume (L)
- Determine Moles of Acid: Write the balanced chemical equation for the neutralization reaction between HCl and the strong base (e.g., NaOH).
  - The balanced equation shows the mole ratio between the acid and the base.
  - Reference Step 2: "Then, using the mole ratio from the balanced neutralization equation, convert from moles of strong base to moles of acid."
  - For HCl and NaOH, the reaction is HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The mole ratio is 1:1.
  - Therefore, moles of HCl = moles of NaOH used.
- Calculate Concentration of Acid: Using the calculated number of moles of HCl and the initial known volume of the HCl solution (used in step 1), calculate the concentration (molarity) of the HCl solution.
  - Formula: Concentration (M) = Moles / Volume (L)

Balanced Neutralization Equation Example:

For titration with Sodium Hydroxide (NaOH):

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

The mole ratio is 1 mole of HCl reacts with 1 mole of NaOH.

Practical Considerations:

Precision: Use accurate glassware (volumetric pipette for the acid, burette for the base).
Multiple Trials: Perform several titration trials (usually 3-5) to ensure consistency and take an average of the concordant (close) titration volumes.
Indicator Choice: The indicator must change color at or near the equivalence point pH of the specific reaction. For a strong acid-strong base titration, the equivalence point is at pH 7.

Calculation Example

Let's say you titrate 25.00 mL of unknown concentration HCl with 0.100 M NaOH.
You perform three trials and get average titration volume of NaOH = 22.50 mL.

Substance	Volume (mL)	Volume (L)	Concentration (M)	Moles
NaOH (Base)	22.50	0.02250	0.100	?
HCl (Acid)	25.00	0.02500	?	?

Calculate moles of NaOH:
Moles of NaOH = Concentration × Volume
Moles of NaOH = 0.100 M × 0.02250 L = 0.00225 moles
Determine moles of HCl:
From the balanced equation HCl + NaOH → NaCl + H₂O, the mole ratio is 1:1.
Moles of HCl = Moles of NaOH = 0.00225 moles
Calculate concentration of HCl:
Concentration of HCl = Moles of HCl / Volume of HCl
Concentration of HCl = 0.00225 moles / 0.02500 L = 0.0900 M

Therefore, the concentration of the hydrochloric acid solution is 0.0900 M.

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