Empirical Formula Of A Compound Worksheets (Page 3 of 6) in pdf

Experimental Procedure

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Safety Note

: Hydrochloric acid is a strong acid that is harmful to the skin and especially to

your eyes. Wear your safety glasses or goggles during the entire procedure to protect your eyes,

and avoid inhaling vapors of HCl during the drying procedure (use a fume hood if possible). The

reaction also produces flammable hydrogen gas (H

), so Bunsen burners should not be used while

the reaction is in progress.

1) Obtain a clean, dry evaporating dish and weigh it to the nearest 0.01 g.

Record this value on the report form.

2) Place a small piece of magnesium ribbon into the evaporating dish and record the mass on the report

form. From the difference in masses, record the mass of magnesium used. Note: The mass of magnesium

should not exceed 0.15 grams, or the product may be difficult to dry (see below).

3) Measure 3 mL of 6 M HCl* in a 10 mL graduated cylinder (or you may use a pipet) and carefully add

the HCl solution to the evaporating dish containing the Mg ribbon (Caution: Vigorous reaction!). Allow

the reaction to proceed until the reaction is complete, giving a clear solution with no magnesium particles

remaining.

4) Place the evaporating dish on an electric hot plate and heat to nearly boiling. Avoid excessive heat

that can cause dangerous splattering of hot HCl!

5) Heat the solution until evaporation of the water is complete. The white, solid product that remains is

magnesium chloride. It is difficult to tell by appearance when the product is completely dry, so we will

use a method called heating to constant weight. When the product appears thoroughly dry, carefully

remove the hot evaporating dish from the hot plate (you may use crucible tongs

to handle the hot

dish), allow it to cool, and record the weight on the report sheet under “first weighing.” Then, place the

dish back on the hot plate and heat for 10 additional minutes. Allow the dish to cool and record the

weight under “second weighing.” If the second weighing agrees with the first weighing, you may

reasonably assume that drying is complete. If the second weighing is less than the first weighing, place

the dish back on the hot plate and heat for 10 more minutes and obtain a third weighing. Repeat this

process until successive weighings agree to within 0.01 g.

An evaporating dish is much better container than a beaker for this process for two reasons:

a) The solid product spreads out more in the evaporating dish as the liquid evaporates, facilitating drying.

b) In a beaker, the vapors of liquid tend to condense on the walls and run back down, inhibiting drying.

6) From your data, calculate the moles of Mg and the moles of Cl in the product. From the number of

moles of each element, determine the empirical formula of magnesium chloride.

7) Wash your evaporating dish with water (the product may be washed down the sink) and return your

equipment to their proper storage locations before leaving the lab.

* The “M” in “6 M HCl” stands for molarity. This is a standard concentration unit in chemistry, and it means

moles of solute per liter of solution, mol/L. One liter of 6 M HCl solution contains 6 moles of HCl.

Empirical Formula Of A Compound Worksheets Page 3