DETERMINATION OF THE FORMULA UNIT OF A COMPOUND

Determination of the formula unit of a compound

Objectives
1. To synthesis a zinc chloride compound.
2. To determine the formula unit of zinc chloride.


Introduction
Chemical compound is a pure chemical substance consisting of two or more different chemical elements which can be separated into simpler substances by chemical reaction chemical compound has a unique chemical structure and defined. They consist of the fixed ratio of atoms are held together in a room setting is defined by chemical bonds. Chemical compound molecule compounds that can be held together by covalent bonds, salt is held together by ionic bonds, intermetallic compounds are held together by metal ties, or a complex held together by a coordinate covalent bond. Another main property of a compound is its chemical composition, which identified only by determining the elements present in the compound. The calculation of the chemical composition of straight ahead. The molecular weight of the empirical formula weight divided by the number of atoms of each element to produce a percentage of each element. This is done for each element in the empirical formula and tabulated as chemical composition. Please note the order is based on electro-negative elements.



Equipment
#Glass Rod
#Crucible
#Wire Gauze
#Tripod Stand
#Porcelain tiles
#Analytical balance
#Bunsen Burner
#10ml of measuring cylinder

Reagent
#HCl 6.0 M
#Zinc Powder

Procedure
1. The crucible was weigh and the exact mass was recorded
2. Approximately 0.25g Zinc Powder was placed in the crucible. The Crucible was weigh again with it’s contain and the exact mass of the zinc powder is determined.
3. 10 ml of 6.0 M HCl solution is added into the crucible containing the zinc powder and the content was stir carefully using glass rod. A vigorous chemical reaction occurs and hydrogen gas was released.
4. Another 5 ml of 6.0 HCl solution was added until the zinc powder dissolve completely.
5. The crucible was placed on a hot plate in the fume cupboard and the heated slowly so that the compound not splatters during heating process.
6. The compound was heated until it completely dry.
7. The crucible was allowed to cool in room temperature, and then weigh it.
8. The crucible was reheat, let it cool at room temperature and then weighs again.
9. The process was repeat 3-4 times until the difference in mass is not more that 0.02 g.
10. The final mass of zinc chloride was determined from the final weigh of the sample. The mass of chlorine in the zinc chloride was calculated.
11. When the mass of zinc and chlorine obtained, the formula unit of zinc chloride is calculated.


Result

Materials                              Mass(g)  
Crucible                              51.92  
Crucible + Zinc Powder      52.18  
Zinc Powder                      0.25

Weight of the crucible + zinc chloride at 1st heating = 52.38 g
Weight of the crucible + zinc chloride at 2nd heating =52.37
Weight of the crucible + zinc chloride at 3rd heating =52.35

Emperical Formula

Element                              Zn                                 Cl  
Mass (g)                             0.25                                0.196  
No of Mole        0.25 / 65 = 0.00385           0.196 / 35 = 0.0056  
Ratio                               1                                  1.5  
Emperical Formula               2                                         3





Question

1. Explain why the content is not weighed while it is still hot?
It is always poor laboratory procedure to weigh something when it is still hot because the hot object will radiate heat in the form of convection currents in the air above the scale. This will alter the air pressure above the object unpredictably and produce unstable readings of the object being massed.

2. Compare the calculated formula unit with the theoretical formula unit. Explain why there is a difference and suggest ways to overcome the error.
The theorecal formula unit will be 1 Zn and 2 Cl so the theoretical formula unit is ZnCl2. But the calculated formula unit is 2 Zn and 3 Cl so the formula unit will be Zn2Cl3. There is a difference because the experiment was done in an open place where there is air presence and we weigh balance also not in a close condition, so there is some error. To overcome this error, the weigh balance should have its cover to prevent the air weighed too.

3. Based on your result, write the equation for the reaction between zinc and HCl.

Zn(s) + 2HCl(l)  H2(g) + ZnCl2





Reference

1. Success Chemistry SPM/Publisher:Oxford Fajar/Written by Tan Yin Toon, Loh Wai Leng and Tan On Tin
2. Ace Ahead stpm text Chemistry Volume 1/ Publisher: Oxford Tan Yin Toon, Loh Wai Leng and Kathirasan Muniandy