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QUESTION

# Explain the use of preparing standard solutions and titrations?

You use a standard solution to determine the concentration of the analyte during a titration.

A standard solution is a solution containing a precisely known concentration of a primary standard.

A primary standard should be a solid with a known formula and a purity of 99.98 %. It must also be stable in air and water-soluble.

Sodium carbonate is a common primary standard base. It is used to prepare standard for the titration of acids.

EXAMPLE

Sodium carbonate (13.25 g) was dissolved in about 150 mL of deionized water in a beaker. The solution was transferred with appropriate washings into a 250 mL volumetric flask. The volume of water was made up to the mark, and the solution was thoroughly shaken to ensure complete mixing.

A 25.00 mL volume of the sodium carbonate solution was pipetted into a conical flask. Methyl orange indicator was added.

The sample required 24.65 mL of a hydrochloric acid solution to completely neutralize it.

Calculate the of the hydrochloric acid.

Solution

Step 1. Calculate the moles of Na₂CO₃.

Moles of Na₂CO₃ = 13.25 g Na₂CO₃ × (1"mol Na₂CO₃")/(105.99"g Na₂CO₃") = 0.125 01 mol Na₂CO₃ (4 + 1 guard digit)

Step 2. Calculate the molarity of the Na₂CO₃ solution.

Molarity = "moles"/"litres" = (0.125 01"mol")/(0.2500"L") = 0.500 05 mol/L

Step 3. Calculate the moles of Na₂CO₃ in the aliquot.

Moles of Na₂CO₃ = 0.02500 L soln × (0.500 05"mol Na₂CO₃")/(1"L soln") = 0.012 501 mol Na₂CO₃

Step 4. Write the balanced chemical equation for the reaction.

Na₂CO₃ + 2HCl → 2NaCl + H₂CO₃

Step 5. Calculate the moles of HCl.

Moles of HCl = 0.012 501 mol Na₂CO₃ × (2"mol HCl")/(1"mol Na₂CO₃") = 0.025 002 mol HCl

Step 6. Calculate the molarity of the HCl.

Molarity = "moles"/"litres" = (0.025 002"mol")/(0.024 65"L") = 1.014 mol/L

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