Zinc Chloride Lab Notes

Zinc Chloride Lab

Empirical Formula Determination

This lab explores how to experimentally determine the empirical formula of zinc chloride by reacting zinc metal with hydrochloric acid and analyzing the resulting product.

• Empirical Formula: The simplest whole-number ratio of atoms in a compound.

• Experimental Question: What is the empirical formula for zinc chloride?

• Prediction: Possible formulas include ZnCl2, ZnCl, and Zn2Cl3.

• Key Principle: Formulas are based on ratios in moles, not mass.

• Example: If 1 mole of Zn reacts with 2 moles of Cl, the empirical formula is ZnCl2.

Lab Plan and Procedure

• Day 1:

  1. Label and record the mass of an empty beaker.

  2. Add and record the mass of zinc to the beaker.

  3. Add some HCl to the beaker (enough to cover the zinc).

  4. Place the beaker in a fume hood overnight to react.

  Data to Record:

  • Volume of HCl used

  • Mass of Zn

  • Mass of beaker

  Observations: Bubbling (gas produced, likely H2).

• Notes: Use the same scale for all mass measurements. Use a beaker for HCl, not a flask. Do not use a lid, as gas will escape.

• Day 2/3:

  1. Add additional HCl if needed to ensure all zinc reacts.

  2. Place the beaker on a hot plate to boil away the liquid.

  3. Record the mass of the zinc chloride product.

  Observations: Zinc is gone; yellowish tint to the liquid.

Calculations

Determining the Empirical Formula

Empirical formulas are based on the mole ratio of elements, not their mass. The following steps outline the calculation process:

1. Measure the mass of zinc used and the mass of zinc chloride produced.

2. Calculate the mass of chlorine in the product:

   • Mass of Cl = Mass of ZnClx - Mass of Zn

3. Convert the masses to moles:

   • Moles of Zn = $\frac{\text{mass of Zn}}{65.38\ \text{g/mol}}$

   • Moles of Cl = $\frac{\text{mass of Cl}}{35.45\ \text{g/mol}}$

4. Find the simplest whole-number ratio of moles Zn : moles Cl.

5. Write the empirical formula based on this ratio.

Example Calculation:

• Mass of Zn: 1.73 g

• Mass of ZnClx: 3.18 g

• Mass of Cl: 3.18 g - 1.73 g = 1.45 g

• Moles of Zn: $\frac{1.73}{65.38} = 0.0264$ mol

• Moles of Cl: $\frac{1.45}{35.45} = 0.0409$ mol

• Ratio: $\frac{0.0409}{0.0264} = 1.55$

• Empirical formula: ZnCl2 (after rounding to nearest whole number)

Interpreting Results and Sources of Error

• If the calculated Cl:Zn mole ratio is too high, possible errors include:

  • Not all zinc reacted (leftover Zn lowers the mass of ZnClx).

  • Added moisture or remaining solution in the beaker increases the apparent mass of ZnClx, making the Cl mass seem too high.

• To correct, ensure all zinc reacts and all liquid is evaporated.

Sample Data Table

The following table summarizes group data for the experiment, showing the mass of zinc, mass of zinc chloride, calculated moles, and empirical formula:

Group	Mass Zn (g)	Mass ZnClx (g)	Moles Zn	Moles Cl	Ratio Cl:Zn	Formula
EX 1	1.73	3.18	0.0264	0.0409	1.55	ZnCl2
EX 2	1.50	2.90	0.0229	0.0400	1.75	ZnCl2
EX 3	2.00	3.80	0.0306	0.0540	1.76	ZnCl2
EX 4	1.20	2.10	0.0184	0.0250	1.36	ZnCl1.4 (not whole number)
EX 5	1.80	3.50	0.0275	0.0500	1.82	ZnCl2

Additional info: Table values are inferred for illustration; actual student data may vary.

Molecular vs. Empirical Formulas

Definitions and Examples

The empirical formula gives the simplest whole-number ratio of atoms in a compound, while the molecular formula gives the actual number of atoms of each element in a molecule.

• Empirical Formula: Simplest ratio (e.g., HO for hydrogen peroxide).

• Molecular Formula: Actual composition (e.g., H2O2 for hydrogen peroxide).

• Relationship: The molecular formula is a whole-number multiple of the empirical formula.

Examples:

• Water: Empirical = H2O, Molecular = H2O

• Hydrogen Peroxide: Empirical = HO, Molecular = H2O2

• Glucose: Empirical = CH2O, Molecular = C6H12O6

Formula:

• $\text{Molecular Formula} = n \times \text{Empirical Formula}$

• Where $n$ is an integer determined by dividing the molar mass of the compound by the molar mass of the empirical formula.

Additional info: Understanding the difference between empirical and molecular formulas is essential for interpreting experimental results and for chemical nomenclature.