TITLE: QUANTITATIVE DETERMINATION OF A CHEMICAL, FORMULA
INTRODUCTION:
According to the atomic theory, in ordinary chemical reactions an atom cannot be divided into smaller parts.
Therefore when two elements combine they must do so in small whole number ratios. For example, when hydrogen and
oxygen react to form the compound water, the ratio of hydrogen atoms to oxygen atoms in a molecule of water is 2 to 1,
never 2.5 to 1 or 1.75 to 1.
It is important to note that one mole of a substance contains 6.02 x 1023 molecules, atoms, or ions
[Avogadro's number]. Therefore 6.02 x 10 23 molecules of water contain 2 x 6.02 x 1023 atoms of hydrogen and 1 x 6.02
x 1023 atoms of oxygen, again a 2:1 ratio.
Purpose: In order to experimentally determine an empirical formula you will combine magnesium with chlorine by
treating the magnesium with hydrochloric acid, You will then calculate the mole ratio of the two elements found in this
product and predict a formula for the compound.
Procedure:
PUT GOGGLES ON NOW!
SAFETY: HYDROCHLORIC ACID CAN
HARM YOUR EYES AND CLOTHING. WEAR AN APRON.
1. Note the number on your test label. It should be your lab station number.
2. Mass the test tube. Record to two decimal places.
3. Get Mg from teacher and add to test tube. Re-mass and record.
4. Attach a test tube holder to the test tube to facilitate handling.
5. Add 3.0M HCl a few drops at a time using the dropper. Run the drops down the side of the test tube
while holding it at a gentle angle. Record your observations. DO NOT DROP ACID STRAIGHT DOWN!
DO NOT ADD A LOT OF ACID AT ONCE. DO NOT POINT THE TEST TUBE AT ANYONE!
6. When the reaction slows down, add a few more drops as instructed above. Continue to add acid until
there is no more bubbling and the magnesium, has disappeared. Be careful not to add too much acid.
[REACTION IS NOW COMPLETE]
7. PAY ATTENTION TO CORRECT TEST TUBE HEATING PROCEDURE AS DEMONSTRATED BY
YOUR TEACHER. YOU WILL LOSE POINTS IF YOU LOSE PRODUCT! DO NOT POINT TEST
TUBE AT ANYONE. HEAT VERY SLOWLY AND CAREFULLY. KEEP THE TEST TUBE MOVING
BACK AND FORTH THROUGH THE FLAME TO PREVENT BOILING THAT WILL CAUSE THE
SOLUTION TO BOIL VIOLENTLY AND SPLASH THE PRODUCT OUT OF THE TUBE. Once the water is
evaporated, allow the tube to cool, then you’ll determine the mass of the product, by massing the tube with
the product and subtracting the mass of the empty tube.
8. When you are finished, wash up. Your product can be washed down the sink. Leave test tube inverted in
your test tube rack to dry.
Empirical Formula Lab
Name_______________________Partner_____________________
Data: 3 pts
Mass of empty test tube
Mass of test tube + magnesium
Mass of test tube and magnesium chloride
(A)
(B)
(C)
Calculations: 3 pts
Mass of magnesium
Mass of magnesium chloride
Mass of chlorine in compound
(B-A)
(C-A)
(C-B)
Using the mass of magnesium and the mass of chlorine in your product determine the percent composition of the
product. 6 pts
Element
mass
Percent = (element mass/total mass) x 100
magnesium
chlorine
Total mass
Using your experimental data find the empirical formula for the compound. Please remember that this is experimental
data and the problem may not work out as nicely as problems done in class.
a.
2 pts Write a formula for the compound with the mass of each element in your product as its subscript.
b.
2 pts Divide the mass subscript by the atomic mass of the element, to convert it to moles.
c.
2 pts Divide both subscripts by the magnesium subscript.
d.
2 pts Rewrite your formula, leave the Chlorine subscript with one decimal place.
What should the formula for magnesium chloride be?
Write a reaction equation for this experiment: 3 pts
Balance it: 1 pts
What type reaction is it? 2 pts synthesis, decomposition, single replacement, double replacement, or combustion.
Conclusion: Write on the back of this paper:
1. 2 pts Purpose: What did you accomplish?(restate the purpose)
2. 2 pts Results: Answer the purpose, compare your formula to that expected.
3. 5 pts Error analysis. What could cause your chlorine subscript to be something higher or lower than 2 as
expected.
Typical errors include, not adding enough acid, adding too much acid, spilling some acid, contaminating
the product with foreign material.