PURPOSE The purpose of this experiment was to determine the molarity

PURPOSE
The purpose of this experiment was to determine the molarity (M) of the unknown Ca2+ solution by converting the 25.00 mL of unknown calcium to solid calcium oxalate monohydrate (CaC2O4.H2O).

BACKGROUND

Gravimetric analysis is a method in which the amount of a specific analyzed ion can be determined through the measurement of its mass. Usually, an insoluble compound would be taken out of a solution by a precipitation reaction. By filtering and drying out the precipitated substance, which can then be massed, the amount of the original ion can be determined. The principle behind gravimetric analysis indicates that once the mass of an atom in a pure compound is identified, it can after be used to find the mass percent of the same atom in a known quantity of an impure mixture. Gravimetric analysis can be classified as one of the most precise and accurate methods of quantitative analysis.

Advantages of Gravimetric Analysis
It is highly accurate and precise, with six-figure accuracy
Involves direct measurement without any form of calibration being required
Provides very little space for instrumental error and does not need a series of standards for the calculation of an unknown.
The methods do not need expensive equipment
The analysis is always fast
Can also be used to determine atomic mass of many different elements

Disadvantages of Gravimetric Analysis
Usually only provides the analysis of a single element, or a group of specific elements, at a time
It is time-consuming
Can causes an experimental error of about 1% due to the solutions made up by hand
Other methods such as spectrophotometry can provide much more efficient results

The four main types of this analysis method are volatilization, precipitation, miscellaneous physical method and electro-analytical. Gravimetric analysis can be used in numerous real-life situations, like chemical analysis where a specific element, its mass or its molarity needs to be found. It can also be used in industrial materials, in the classification of elements, in the calibration of instruments, and in elemental the analysis of inorganic compounds. In many schools, the gravimetric test is commonly used to determine the amount of water containment. By heating the water and weighing it before and after the heating process. The weight difference occurs due to the additional energy molecules that water received during the heating process, changing the state of some of the particles of water into gas molecules.
If any of the precipitation takes longer to form because of the small rate of reaction. One of the solutions is to gradually increase the temperature to produce a slow chemical reaction. Which maintains the solute at an effectively constant level. Forming pure crystals of product where the particle growth increases the rate of reaction, resulting in larger precipitate particles. This process is called homogeneous precipitation, in which the precipitating agent is synthesized in the solution.
Two common methods are used for homogeneous precipitation. If the precipitate’s solubility is pH-dependent, where the pH can be increased or decreased chemically. For example, the hydrolysis of urea is a source of OH–. Because the hydrolysis of urea is temperature-dependent, we can use temperature to control the rate of hydrolysis and the rate of precipitate formation. In the second method, the precipitate is generated by a chemical reaction. For example, Pb2+ is precipitated homogeneously as PbCrO4 by using bromate, BrO3–, to oxidize Cr3+ to CrO42–.
After all the elements have been precipitated, vacuum filtration is used to separate the solid that was formed from the fluid reaction mixture. The mixture is after poured through the funnel and the filter paper into the flask below. By the vacuum filtration process, the fluid is entirely carried out from the precipitation. Vacuum filtration can be used in the filtration of very small particles, drying them out faster for a quicker analysis. It is also known to be a fast process as requires a very minimal amount of time.

Equation 1:
H+(aq) + C2O42- (aq) ? HC2O4-(aq)
Oxalate
At the beginning of the experiment there were enough oxalate ions for the calcium to react.
Equation 2:

(H2N)2CO(s) + 3H2O(l) ? CO2(g) + 2NH4+(aq) + 2OH-(aq)
Urea

Urea was added to the solution and heated at a constant temperature for approximately 30 minutes. The urea was then decomposed forming carbon dioxide. The ammonia and OH- ions that were created, slowly neutralized the H+ ions.

Equation 3:

HC2O4-(aq) + OH-(aq) ? C2O42- (aq) + H2O(l)

While the amount of H+ decreased the reaction took place in the reversed reaction and slowly released small amounts of C2O42-. As small amounts of C2O42- were formed, slowly bound with Ca2+ forming calcium oxide. Heated was needed and continuously applied to the reaction to decompose urea, which then formed an insoluble complex, precipitating the solution.

Equation 4:

Ca2+(aq) + C2O42- (aq) + H2O(l) ? CaC2O4.H2O(s)

Finally, at one stage all the H+ got completely neutralized and formed into calcium. The solution went from being acidic to basic and also changed its colour from pink to yellowish. After that, the precipitation was filtered out using the vacuum filtration.

MATERIALS
Filter flask
Filter paper
Buchner funnel
Volumetric pipette
Loading balance
Crucible tongs
Hot plate
50 mL beaker
100 mL beaker
250 mL beaker
Distilled water
Ice-cold deionized water
Drying oven
25.00 mL of the unknown Ca2+ solution
75 mL of 1.0 M HCl
Methyl Red
Boiling Chips
25 mL of Ammonium Oxalate
15g of Urea

METHOD

75 mL of 0.1M HCl was poured into a 250 mL beaker.
25.00 mL of the unknown Ca2+ solution was precisely measured with a volumetric pipette and poured into the existing HCl solution.
Approximately 2 g of boiling chips were added to the beaker to make the solution boil more calmly and faster.
25 mL of Ammonium Oxalate was added to the solution while stirring with a glass rod.
15 g of urea were added to change the solution pH.
Five drops of the indicator methyl red were added, resulting in a pink color solution.
The solution was kept at a constant temperature for 30 minutes until the precipitation was formed.
The solution was removed from the hot plate and allowed it to cool.
The warm solution was filtered throughout the preweighed funnel and filter paper using the vacuum filtration.
Enough ice-cold deionized water was used to rinse the beaker until all the precipitate was transferred to the funnel.
The funnel with the sample on the filter paper was placed in the drying oven for five days.
Finally, the funnel with the precipitate solution was properly weighed.

RESULTS

Data for Gravimetric Determination of Ca2+

Volume (mL)
Volume unknown Ca2+ solution
25.00 mL

Mass (g)
Mass of funnel, filter paper and solid CaC2O4.H2O
106.00 g
Mass of empty funnel and filter paper
105.34 g
Mass of CaC2O4.H2O
0.66 g

Mass of CaC2O4.H2O = 0.66g
Molar mass of CaC2O4.H2O = 146.12 g/mol
Number of moles of CaC2O4.H2O = 0.66g ÷ 146.12g/mol = 4.5×10-3 moles of CaC2O4·H2O
Number of moles of Ca2+ = 4.5×10-3 moles of Ca2+
Volume of unknown Ca2+ solution = 25.00 mL or 0.025 L
Molarity of the unknown Ca2+ solution = 4.5×10-3 mol ÷ 0.025 L = 0.18 M

DISCUSSION

During the experiment, urea was added to the acidic HCl solution to change its pH. For the precipitation to form, a very slow boiling process was required to supreme the dissociation of ammonia oxalate. After the experiment was completed and the precipitate was weighed, the mass of the unknown calcium was found to be 0.66 g. Its molarity was also calculated by dividing the number of moles and the volume getting 0.18 M as the final result.

CONCLUSION

It is evident that the gravimetric analysis is really helpful when it comes to analyzed many compounds in different solutions. Gravimetric analysis is not only found to be inexpensive but also is a quick method to measure the properties of the elements. After the experiment was completed and the precipitate was formed and adequately weighed, it was possible to determine the exact molarity of the calcium solution.