AP Chemistry
4.8 Introduction to Acid-Base Reactions
4.7 Types of Chemical Reactions
4.6 Introduction to Titration
4.5 Stoichiometry
A tablet contains calcium carbonate, \(\text{CaCO}_3\), and an acid-insoluble binder that contains no calcium. A student analyzes separate, accurately weighed portions of the powdered tablet by gravimetric precipitation and by acid-base back titration. The reactions used are
\[ \text{CaCO}_3\text{(s)}+2\text{H}^+\text{(aq)}\rightarrow \text{Ca}^{2+}\text{(aq)}+\text{CO}_2\text{(g)}+\text{H}_2\text{O(l)} \]
\[ \text{Ca}^{2+}\text{(aq)}+\text{C}_2\text{O}_4^{2-}\text{(aq)}+\text{H}_2\text{O(l)}\rightarrow \text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O(s)} \]
\[ \text{H}^+\text{(aq)}+\text{OH}^-\text{(aq)}\rightarrow \text{H}_2\text{O(l)} \]
In the gravimetric trial, the student dissolves the \(\text{CaCO}_3\) in acid, removes the binder by filtration, and quantitatively precipitates \(\text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O}\). In the back-titration trial, the student adds a known excess of standardized \(\text{HCl}\) and determines the excess acid using standardized \(\text{NaOH}\). Some \(\text{CO}_2\) remains dissolved after visible bubbling stops. A brief gentle-heating step can expel the dissolved \(\text{CO}_2\) without loss of \(\text{HCl}\). Which combined procedure and calculation will produce accurate purity values from both trials?
\[ \text{CaCO}_3\text{(s)}+2\text{H}^+\text{(aq)}\rightarrow \text{Ca}^{2+}\text{(aq)}+\text{CO}_2\text{(g)}+\text{H}_2\text{O(l)} \]
\[ \text{Ca}^{2+}\text{(aq)}+\text{C}_2\text{O}_4^{2-}\text{(aq)}+\text{H}_2\text{O(l)}\rightarrow \text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O(s)} \]
\[ \text{H}^+\text{(aq)}+\text{OH}^-\text{(aq)}\rightarrow \text{H}_2\text{O(l)} \]
In the gravimetric trial, the student dissolves the \(\text{CaCO}_3\) in acid, removes the binder by filtration, and quantitatively precipitates \(\text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O}\). In the back-titration trial, the student adds a known excess of standardized \(\text{HCl}\) and determines the excess acid using standardized \(\text{NaOH}\). Some \(\text{CO}_2\) remains dissolved after visible bubbling stops. A brief gentle-heating step can expel the dissolved \(\text{CO}_2\) without loss of \(\text{HCl}\). Which combined procedure and calculation will produce accurate purity values from both trials?
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