---
title: "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?"
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url: "https://nerd-notes.com/ubq/120086/"
date_modified: "2026-08-21T08:41:23+00:00"
---

# 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?

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?

- **A.** Wash the oxalate precipitate, dry it to constant mass, and use \(n(\text{CaCO}_3)=n(\text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O})\). Titrate the acidic solution immediately and use \(n(\text{CaCO}_3)=\dfrac{n_{\text{initial}}(\text{HCl})-n(\text{NaOH})}{2}\).
- **B.** Dry the oxalate precipitate to constant mass without washing it and use \(n(\text{CaCO}_3)=n(\text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O})\). Expel \(\text{CO}_2\), cool the solution, and use \(n(\text{CaCO}_3)=\dfrac{n_{\text{initial}}(\text{HCl})-n(\text{NaOH})}{2}\).
- **C.** Wash the oxalate precipitate, dry it to constant mass, and use \(n(\text{CaCO}_3)=n(\text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O})\). Expel \(\text{CO}_2\), cool the solution, and use \(n(\text{CaCO}_3)=\dfrac{n_{\text{initial}}(\text{HCl})-n(\text{NaOH})}{2}\).
- **D.** Wash the oxalate precipitate, dry it to constant mass, and use \(n(\text{CaCO}_3)=n(\text{CaC}_2\text{O}_4\cdot\text{H}_2\text{O})\). Expel \(\text{CO}_2\), cool the solution, and use \(n(\text{CaCO}_3)=n_{\text{initial}}(\text{HCl})-n(\text{NaOH})\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120086/*