# Explain the use of preparing standard solutions and titrations?

Aug 24, 2014

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 solutions 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 molarity of the hydrochloric acid.

Solution

Step 1. Calculate the moles of Na₂CO₃.

Moles of Na₂CO₃ = 13.25 g Na₂CO₃ × $\left(1 \text{mol Na₂CO₃")/(105.99"g Na₂CO₃}\right)$ = 0.125 01 mol Na₂CO₃ (4 significant figures + 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 × $\left(0.500 05 \text{mol Na₂CO₃")/(1"L soln}\right)$ = 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 × $\left(2 \text{mol HCl")/(1"mol Na₂CO₃}\right)$ = 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