Modified Empirical Lab Procedure for Chemistry 121

Page3

Lab 8. Empirical Formula LAB

Introduction
In this lab you will combine zinc metal with hydrochloric acid to form zinc chloride. Hydrogen gas (H2) will be given off as a by-product. You will use your data to calculate the empirical formula of zinc chloride and compare it to the accepted value.

The chemical reaction is shown below.

Zn(s) + HCl(aq) ? ZnxCly(aq) + H2(g)

After all the Zn has reacted, we apply heat to evaporate all the water (and remaining HCl).

Procedure 1. Weigh an empty and dried evaporating dish1 to the nearest 0.01g and record the mass of the dish in the data sheet (2). Place approximately 0.4 to 0.5 grams of granular zinc metal with a spatula in the dish. Record the total mass to the nearest 0.01g (1). Calculate the mass of the zinc (3). 2. In a hood, slowly and with constant stirring, add 15.0 mL of 6 M hydrochloric acid to the dish containing the zinc. A vigorous reaction will ensue. ( Caution: No flames are permitted while this reaction is taking place because hydrogen gas is explosive.) If any undissolved zinc remains after the reaction ceases, add an additional 2 mL of the acid to make all zinc reacted. [Do steps 1 and 2 at the beginning of the lab] 3. Set up a steam bath as shown in figure 1 using a 250 mL beaker, and place the evaporating dish from step 2 on the steam bath2,3. Heat the evaporating dish carefully to reduce the volume of the solution to one fourth or less.

Alternatively you can use a hot plate instead of a Bunsen burner to do the same [see figure 2].

Note the position of

evaporating dish

Figure 1. Steam bath with Bunsen burner Figure 2. Steam bath with hotplate

4. Remove the steam bath, heat the dish containing reduced volume of solution on a wire gauze directly on a hot plate [see figure 3]. Control the heating rate if necessary to obtain a solid white mass. [Your objective is to remove all water].

4. Allow the dish to cool to room temperature and weigh it. Record the mass (4a).

5. To ensure that the water has completely evaporated, reheat the dish and compound for 5 more minutes; cool it and reweigh (4b). Continue if necessary one more time to obtain a constant mass [two masses should not differ more than 0.02 g]. 6. Calculate mass of zinc chloride (5). Calculate mass of chlorine in zinc chloride (6).

7. Wash out the evaporating dish and clean up your lab station.

8. Complete the calculations of the empirical formula.

C:UsersgpaulDesktopIMG_1510.JPG

Figure 3. Final drying to a solid product

1Clean an evaporating dish and dry with a paper towel.

2Note the placement of evaporating dish on steam bath (see picture 2)

3Caution: never let water in the beaker (steam bath) boiled off completely; if necessary turn off flame or heat, allow the beaker to cool, and add more water to maintain a steam bath.

Data Sheet: Chemical Formula

1. Mass of evaporating dish and zinc _____76.541 g

2. Mass of evaporating dish _____76.507 g

3. Mass of zinc ____________g

4. Mass of evaporating dish and zinc chloride:

(a) First weight _____77.670 g

(b) Second weight _____77.658 g

5. Mass of zinc chloride _______________ g

6. Mass of Cl in zinc chloride ________________ g

7. Empirical Formula calculation:

Mole of zinc:

Mole of chlorine atom:

Mole ratio:

Empirical formula for zinc chloride ___________________

Determination of Empirical Formula (Zinc chloride) [6.30 min]

reaction of excess Zn with HCl, excess Zn removed, dried, etc.

data available in video to calculate EF.

Post Laboratory Questions:

1. Could the empirical formula of zinc chloride be determined using this experiment if HCl had been the limiting reactant? Explain.

2. Define empirical formula. What is the empirical formula of a compound?

3. A compound is 26.6% potassium, 35.3% chromium, and 38.1% oxygen. What is the empirical formula of the compound?

4. A piece magnesium metal (Mg) was combusted with excess O2 in a crucible (ceramic container to heat at high temperatures) produce magnesium oxide as the only product, according to the reaction shown below.

Mg(s) + O2(g) ? MgxOy(s)

Use the mass data provided to calculate the empirical formula of magnesium oxide.

Mass of empty crucible: __ 21.040 g

Mass of crucible + Mg: __ 21.601 g

Mass of Mg: _______________

Mass of crucible + magnesium oxide __ 21.974 g

Mass of magnesium oxide: _______________

Mass of O (in magnesium oxide): _______________

Determine the empirical formula (show work)

Empirical formula of magnesium oxide: _______________

Determination of empirical formula (MgO):

https://www.youtube.com/watch?v=1Mi7_O07k_M [expt]

Pre-Laboratory Questions:
1. What does the empirical formula of a compound represent?

2. If a 1.271 gram sample of aluminum metal is heated in chlorine gas, the mass of aluminum

chloride produced is 6.280 gram. Calculate the empirical formula.

3. A compound has the following percentages by mass: copper, 33.88%, nitrogen,

14.94%, oxygen, 51.18%. Determine the empirical formula.