Acid-base Titration

2 cm3

=25.23 cm3

Na2CO3 (aq) + H2SO4 (aq) ›Na2SO4 (aq)+ CO2 (g)+H2O (l)

0.0500M ? M

25.00cm3 25.23cm3

No. of moles of sodium carbonate

=Molarity of solution ×Volume of solution

=0.0500 M ×0.0025 dm3

=0.000125mol

According to the equation,

mole of Na2CO3 required 1 mole of H2S04 for complete neutralization.

?No of moles of H2SO4 = 0.000125mol

Concentration of H2SO4 solution

= No. of moles of H2SO4

Volume of solution

= 0.000125mol

0.002523dm3

=0.0495mol/dm3

Question:

. In standardization of sulphuric acid solution with standard sodium sodium carbonate solution, methyl orange is used as the indicatior to show colour change for the end point. Why do we use methyl orange?

Ans: If we use phenolpthlein as indicator, the solution will turn from

pink to colourless, however, the solution which is neutral or acidic remain as colourless, therefore it is not easy for us to determine the end point,we cannot obtain an accurate volume of acid used. But when we use methyl orange, the solution will turn from yelow to pale orange. When the solution is neutral, it remain pale orange, when it is acidic, it will turn red, therefore, it is easy for us to determine the end point, so that we can obtain a more accurate result.

(p.t.o. to continue)

For strong acid (H2SO4) with weak base (Na2CO3), the pH at the equivalence point is always less than 7 because of the acidity of the conjugate acid of the weak base. And from the titration curve for strong acid with weak base, we know that the pH range of methyl orange (3.1-4.4) is nearer to the equivalence point than the pH range of phenolpthalein (8.3-10.0), therefore it is more suitable for us to use methyl orange for standardization of sulphuric acid solution with standard sodium carbonate solution.

Titration curve for strong acid with weak base

2. What is the difference between the end point and the equivalence point in volumetric analysis?

Ans.: Equivalence point of the reaction is the point at which the original acid (or base) in the solution has been exactly consumed by the titrant base (or acid). End point is the point at which indicator changes its colour, it occurs when the stoichiometric amount of acid has been added to the base. However as it is not easy for us to obtain an accurate value for end point, therefore it will be a little bit different from the equivalence point.

3. Why can phenolphthalein and methyl orange be used as the indicator when sodium hydroxide solution is titrated with standard sulphuric acid solution?

Ans.: Because according to the titration curve for strong acid with strong base, the pH range for methyl orange (3.1-4.4) and the pH range of phenolpthalein (8.3-10.0) is very near to the equivalence point, therefore we can use both phenolpthalein and methyl orange as the indicator when sodum hydroxide solution is titrated with standard sulphuric acid solution.

Titration curve for strong acid with strong base