Question

The experiment is Gravimetric Determination of Ca2+. Please answer as many questions as you can.

Write all the balanced equations involved in this experiment.

What is the purpose of urea? Can another base like NH4OH be used instead? Why?

Why was the reaction heated (to gentle boil) after adding urea?

What is the molarity of concentrated HCl (12 M)? How would you prepare 100 mL of O.1 M dilute HCl solution from it?

What is methyl red? Why was it used?

An aqueous solution of CaC2O4 would be basic, acidic or neutral? Why?

The product on the crucible was rinsed with cold water. What is the purpose of the rinse? What will happen if hot/warm was used?

Suggest a practical use of this lab (gravimetric determination of Ca) in real world?

What is water of hydration? How/Why does it become part of a crystalline inorganic molecule?

What happens when CaC2O4.H2O is heated at >500 0C for long time?

A 10.00 mL solution containing Cl- was treated with excess AgNO3 to precipitate 0.4368 g of AgCl. What was the molarity of Cl- in the unknown? FW of of AgCl is 143.321 g/mol.

Answer 1

1. Calcium ion can be determined by precipitation with oxalate in basic solution to form calcium oxalate monohydrate. The basic solution for precipitation is achieved by thermal decomposition of urea.

The balanced equations involved in the experiment are

Ca²⁺ + C₂O₄^2- + H₂O → CaC₂O₄.H₂O

(H₂N)₂CO + 3 H₂O → CO₂ + 2 NH₄⁺ + 2 OH^-

2. Calcium oxalate is soluble in acidic solution. In order to make homogeneous precipitation, the pH of the solution should gradually adjust to more basic by thermal decomposition of urea.

We can not use NH₄OH instead of urea because pure crystals of product will not be obtained if the precipitation is fast. NH₄OH readily dissociates in aqueous solution and produces ammonium ions. We won’t be able to increase the pH of the solution gradually.

3. The reaction heated (to gentle boil) after adding urea to increase the degree of hydrolysis of urea and therefore increase the formation of ammonia gradually. The formation and hydrolysis of ammonia slowly increase the pH of solution that is sufficient to generate free oxalate and precipitate calcium oxalate.

4. Use the equation, N₁V₁= N₂V₂

0.1M x 100 mL /12M = 0.83 mL is needed.

5. Methyl red is (2-(N,N-dimethyl-4-aminophenyl)azobenzenecarboxylicacid). Methyl red 4is a pH indicator changes color when the pH changes between 4.4 and 6.; it is red in pH under 4.4, yellow in pH over 6.2, and orange in between. In the experiment precipitation occurs at this range of pH. So it will be easy to monitor the reaction.