Question

How can I make a back titration calculation?

Answer 1

A back titration is a titration method where the concentration of an analyte is determined by reacting it with a known amount of excess reagent. The remaining excess reagent is then titrated with another second reagent. It is called back titration because it is not carried out with the solution whose concentration is required to be known as in the case of normal or forward titration, but with the excess volume of reactant which has been left over after completing reaction with the analyte.

Complete step by step answer:
Back titration works is following manner:
The substance or solution of unknown concentration is made to react with known volume and concentration of intermediate reactant solution. The reaction goes past the equivalence point. The amount of intermediate reactant is in excess of that required for completing reaction with analyte.
After completing the reaction with analyte, the resulting solution contains excess intermediate reactant saturated with known volume and concentration of titrant. Is subscripts 1 and 2 denotes intermediate reactant then
${{\text{N}}_{1}}{{\text{V}}_{1}}={{\text{N}}_{2}}{{\text{V}}_{2}}$
$0.5\text{V}=\text{0}\text{.5}\times \text{50}$
Volume of excess HCl, $\text{V}=\text{50ml}$
Determination of excess volume of intermediate reactants allows us to determine the volume or meq of intermediate reactant which reacts with analyte. This, in turn, lets us determine the amount of analysis.
Meq of chalk $=\ \text{meq of Hcl used for chalk}=\text{75}$
${{g}_{E}}\times 1000=\dfrac{2\times 1000\text{g}}{(40+12+3\times 16)}=75$
$\text{g}=\text{3}\text{.75gm}$
Total was 4 gm impurity $=4.375=0.25\text{g}$
Chalk contained $0.25\text{g}$ of impurities

Additional Information:
Complex ometric titrations relyon the formation of a complex between the analyte and titrant. In general they require specialized complexometric indicators that form weak complexes with the analyte. Gas phase titrations are titrations done in the gas phase, specifically as methods for determining reactive species by reaction with an excess of some other gas, acting as the titrant. In an acid base titration, the titration curve represents the strength of the corresponding acid and base. For a strong acid and a strong base, the curve will be relatively smooth and very steep near the equivalence point.

Note: The analyte may contain impurities which may interfere with direct titration. We can filter out the impurities before the excess reactant is titrated and thus avoid this situation. The analyte reacts slowly with titrant in direct or forward titration. The reaction with the intermediate reactant can be speeded up and reaction can be completed by heating. Weak acid-weak base reactions can be subjected to back titration for analysis of solution of unknown concentration.