Question

Bone was dissolved in hydrochloric acid, giving 50.0 \(\mathrm{mL}\) of solution containing calcium chloride, \(\mathrm{CaCl}_{2}\). To precipitate the calcium ion from the resulting solution, an excess of potassium oxalate was added. The precipitate of calcium oxalate, \(\mathrm{CaC}_{2} \mathrm{O}_{4}\), weighed \(1.437 \mathrm{~g}\). What was the molarity of \(\mathrm{CaCl}_{2}\) in the solution?

Short answer

The molarity of the CaCl2 solution is 0.2244 M.

Step-by-step solution

Understand the Reaction Involved

The calcium ions (Ca^{2+}) first came from CaCl_2 in the solution. Upon the addition of excess potassium oxalate, Ca^{2+} reacts to form calcium oxalate (CaC_2O_4) precipitate. The reaction can be summarized as:\[CaCl_2 (aq) + K_2C_2O_4 (aq) \rightarrow CaC_2O_4 (s) + 2KCl (aq)\]

Determine the Moles of Calcium Oxalate Formed

The mass of CaC_2O_4 precipitate formed was given as 1.437 grams. First, calculate the moles of CaC_2O_4 using its molar mass. The molar mass of CaC_2O_4 is approximately 128.10 g/mol.\[\text{Moles of } CaC_2O_4 = \frac{1.437 \, \text{g}}{128.10 \, \text{g/mol}} = 0.01122 \, \text{mol}\]

Relate Moles of Calcium Chloride and Calcium Oxalate

Since CaCl_2 produces one Ca^{2+} ion which reacts to form one mole of CaC_2O_4 , the moles of CaCl_2 are equal to the moles of CaC_2O_4 formed. Hence, moles of CaCl_2 is also 0.01122 mol.

Calculate Molarity of CaCl2 Solution

Molarity (M) of a solution is defined as the number of moles of solute divided by the volume of the solution in liters.The volume of the solution is 50.0 mL, which is 0.0500 L.\[M = \frac{\text{moles of } CaCl_2}{\text{volume of solution in L}} = \frac{0.01122 \, \text{mol}}{0.0500 \, \text{L}} = 0.2244 \, \text{M}\]

Finalize the Answer

The molarity of the CaCl_2 solution is determined by the calculation. Therefore, the molarity of the CaCl_2 solution is 0.2244 M.

Key concepts

Calcium chloride

Calcium chloride (CaCl₂) is a chemical compound composed of calcium and chlorine. It is commonly encountered in its solid state as a white crystalline substance and is highly soluble in water, forming a clear solution.

In chemical reactions, such as the one outlined in this exercise, calcium chloride often provides calcium ions (Ca²⁺) that can engage in further reactions.

• It is used in a variety of applications, including as a de-icer, in drying operations, and as a firming agent in food.
• In this scenario, calcium chloride serves as a source of calcium ions in a solution that will undergo a chemical transformation.

Understanding the role of calcium chloride in this context can help clarify its utility in various applications and experiments.

Precipitation reaction

A precipitation reaction occurs when two soluble substances in solution react to form an insoluble solid, known as a precipitate.

In the case of the exercise provided, when calcium ions (Ca²⁺) from calcium chloride are mixed with excess potassium oxalate, a precipitation reaction occurs.

• This results in the formation of calcium oxalate (CaC₂O₄) as the solid precipitate.
• The general reaction is represented as: \[CaCl_2 (aq) + K_2C_2O_4 (aq) \rightarrow CaC_2O_4 (s) + 2KCl (aq)\]

This indicates that calcium oxalate is insoluble in the solution and settles out as a solid.

Calcium oxalate

Calcium oxalate ( CaC₂O₄ ) forms as an insoluble salt during the precipitation reaction. Recognized by its crystalline structure, it is commonly found in nature as a mineral.

In the context of the provided exercise, calcium oxalate precipitates out of solution when calcium ions ( Ca²⁺ ) and oxalate ions ( C₂O₄²⁻ ) meet.

• It is this formation of a solid precipitate that allows us to calculate the amount of calcium that was originally present in the solution.
• By measuring the mass of this precipitate, one can back-calculate to find out the molarity of the original calcium chloride solution.

This approach is commonly used in quantitative analysis.

Hydrochloric acid dissolution

Hydrochloric acid (HCl) is a strong acid that can completely dissociate in water, releasing hydrogen ions and chloride ions into the solution.

When hydrochloric acid dissolves bone or other structures, it effectively releases calcium ions ( Ca²⁺ ) from compounds such as calcium phosphate.

• The dissolution of bone in HCl in this exercise leads to the formation of a solution of calcium chloride ( CaCl₂ ).
• This process is crucial for solubilizing the calcium, making it available for further chemical reactions, like the precipitation with potassium oxalate.

Understanding this dissolution process is essential for appreciating how the original calcium becomes part of the solution amenable to reaction.