In some laboratory analyses, the preferred technique is to...

In some laboratory analyses, the preferred technique is to dissolve a sample in an excess of acid or base and then "back-titrate" the unreacted acid or base with a standard base or acid. To assess the purity of a sample of $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ you dissolve a 0.475 -g sample of impure $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ in aqueous KOH. $$\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{KOH}(\mathrm{aq}) \rightarrow 2 \mathrm{NH}_{3}(\mathrm{aq})+\mathrm{K}_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)$$ The $\mathrm{NH}_{3}$ liberated in the reaction is distilled from the solution into a flask containing $50.0 \mathrm{mL}$ of 0.100 M HCl. The ammonia reacts with the acid to produce $\mathrm{NH}_{4} \mathrm{Cl},$ but not all of the HCl is used in this reaction. The amount of excess acid is determined by titrating the solution with standardized NaOH. This titration consumes $11.1 \mathrm{mL}$ of 0.121 M NaOH. What is the weight percent of $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ in the 0.475 -g sample?

In some laboratory analyses, the preferred technique is to dissolve a sample in an excess of acid or base and then "back-titrate" the unreacted acid or base with a standard base or acid. To assess the purity of a sample of $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ you dissolve a 0.475 -g sample of impure $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ in aqueous KOH. $$\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{KOH}(\mathrm{aq}) \rightarrow 2 \mathrm{NH}_{3}(\mathrm{aq})+\mathrm{K}_{2} \mathrm{SO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)$$
The $\mathrm{NH}_{3}$ liberated in the reaction is distilled from the solution into a flask containing $50.0 \mathrm{mL}$ of 0.100 M HCl. The ammonia reacts with the acid to produce $\mathrm{NH}_{4} \mathrm{Cl},$ but not all of the HCl is used in this reaction. The amount of excess acid is determined by titrating the solution with standardized NaOH. This titration consumes $11.1 \mathrm{mL}$ of 0.121 M NaOH. What is the weight percent of $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ in the 0.475 -g sample?

Key Concepts

Weight Percent (Percent Purity) Calculation

Weight percent or percent purity is a way of expressing the concentration of a substance in a mixture by comparing the mass of the target component to the total mass of the sample. This calculation involves determining the mass of the pure component, often through titration or other quantitative analysis, and dividing it by the total sample mass, then multiplying by 100 to obtain a percentage, providing a measure of the sample's purity.

Acid-Base Titration

Acid-base titration is a common laboratory method used to determine the concentration of an acid or base in a solution. In this procedure, a solution of known concentration (the titrant) is added to a known volume of a solution with unknown concentration until the reaction reaches its equivalence point, as indicated by a suitable indicator or instrument, thereby allowing the calculation of the unknown concentration.

Back Titration

Back titration is an analytical technique used when the analyte is difficult to titrate directly. In this method, an excess amount of a standard reagent is added to the sample, and the remaining unreacted excess is titrated with another standard solution. This method can improve accuracy and is especially useful when the endpoint of the direct titration is not clear or when the reaction of interest is slow or incomplete.

Stoichiometry in Chemical Reactions

Stoichiometry involves the quantitative relationship between reactants and products in a chemical reaction. By using the balanced chemical equation, one can determine how many moles of one substance correspond to the moles of another. It forms the basis of calculating unknown concentrations or masses in a reaction by relating them via mole ratios.

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