SCH 3U1 Unit 1: Experiment
: The Law of Definite Proportions
Introduction:
When magnesium is burned in air , there is an increase in mass.
The magnesium combines with oxygen in the air to form a new ionic substance called
magnesium oxide.
The PROBLEM:
a) Exactly what is the experimental % by mass of Mg in magnesium oxide
b) Exactly what is the theoretical % by mass of Mg in magnesium oxide
c) Is the formula of magnesium oxide - MgO ?
The Experiment:
1. Determine the mass of a clean, dry crucible and lid. Record this mass.
2. Obtain a strip of magnesium from the front desk and clean it with some sand paper. Find the mass of your
magnesium and record it.
Coil the Mg ribbon (not too tight) and place it on the bottom of the crucible
3. Set the crucible and contents on a clay triangle with the lid slightly ajar so fresh air can enter but no ash can escape.
4. Heat the crucible with a hot flame for about 5-7 min. Allow to cool then reweigh.
Heat the crucible again for 1-2 min then cool and reweigh.** heating to constant mass !
Data/Calculations:
1. Calculate the mass of the ash (magnesium oxide) that is left in the crucible after heating
2. Calculate the experimental % by mass of Mg in your magnesium oxide (the ash)
What would the % oxygen be in your magnesium oxide?
3. Use your periodic table and calculate the theoretical % by mass of Mg in magnesium oxide.
4. Calculate the % error in your experiment
CONCLUSIONS: How does your experimental result compare to the theoretical result ? Therefore??
This lab requires a formal lab report including
a) The purpose/problem
b) all data ( presented correctly) c) all calculations (done with proper format)
d) conclusions (answer purpose/comments + % error if possible) e) related questions
Questions:
1.Calculate
a) the % oxygen in dinitrogen tetraoxide
c) the % nitrogen in ammonium nitrate
e) Do practice problems 7 & 8 on pg 204
g) Do Section Review problems 4-7 on pg 206
b) the % sulfur in stannic sulfate
d) the % carbon in C6H8N2O2S
f) Do practice problems 3 & 4 on pg 201
2. The label has fallen off a bottle in the storeroom that was known to be either nickel(II) phosphate, nickel(II)
nitrate or nickel(II) sulfate. It was analyzed and found to contain 32% nickel. Which compound was in the
bottle?? (show all your work)
3. An oxide of mercury was discovered but which one? A chemical mystery!! To determine its identity an
experiment similar to yours was done.
An empty testtube weighed - 16.286 g
The testtube with the mercury oxide - 17.267 g
The testtube was heated and the oxygen was driven out !!!
The testtube with the mercury- 17.196 g
a) Calculate the mass of i) the mercury oxide ii) the mercury.
b) Calculate the experimental % by mass of mercury in this oxide.
c) Using your periodic table calculate the theoretical % by mass of Hg in mercurous oxide and mercuric oxide
then identify which oxide of mercury it was in this experiment.
SCH 3U1 Unit 1: Experiment
: To find the % Composition of an oxide of copper & Identify it
1. INITIAL WEIGHING
Weigh out about 2 g of the black oxide. Record the exact mass of the oxide.
Place the weighed oxide into a 250 mL beaker.
2. CONVERTING THE OXIDE TO THE SOLUBLE SULFATE
Add about 150 mL of distilled water to the beaker. Now gradually add about 50 mL of conc sulfuric acid, stirring
continually with a glass rod. Stir until all ( or nearly all ) of the black oxide has dissolved.
Record any observations.
N.B. Always add acid to water, never the other way around. The reaction of water with acid is very exothermic so the
water quickly changes to steam and splatters the acid on YOU.
3. REDUCING THE SULFATE TO METALLIC COPPER
Add, a little at a time, about 6 g of 20-mesh zinc to the blue coloured sol'n. Stir and swirl as you add it. The
mixture should bubble vigorously. If not, add water until it does. NEVER add zinc so fast that the
effervescence becomes too vigorous. Cover with a glass plate to prevent any splashing. As the reaction
subsides, stir until all the effervescence ceases. If the sol'n is still blue, add more zinc, a little at a time , until
the blue colour is all gone.
4. WASHING AND FILTERING THE COPPER
When all the blue colour is gone, allow the metallic copper to settle to the bottom . Decant the supernatant liquid, taking
care not to lose any copper. Add about 50 mL of distilled water to the beaker.
Fold a piece of filter paper and insert it into a funnel. Dampen slightly. Carefully pour in the contents of the beaker into
the filter paper. Rinse out the beaker with a small portion of water (about 25 mL), and add the washings to the filter
funnel.
Remove the filter paper and place it in an evaporating dish in the drying oven overnight to dry.
5. CALCULATIONS and CONCLUSION
Scrape the dry copper off the filter paper onto a tared weighing paper and find the mass of your copper. This must be
done very carefully so as not to lose any copper.
(A) Calculate the experimental % copper and % oxygen in your black oxide
(B) Calculate the theoretical % copper in
i) copper suboxide - Cu4O
ii) cuprous oxide - Cu2O
iii) cupric oxide
- CuO
iv) copper peroxide - CuO2
What conclusion can be drawn about your oxide used in this experiment ?
**this lab requires a formal lab write up with all the headings etc including the questions below!
6. The EQUATIONS:
(just for your informatiion)
a) copper oxide (s) + H2SO4 (aq) → CuSO4 (aq) + H2O (l)
b)
Zn (s)
+ H2SO4 (aq)
→ ZnSO4 (aq) + H2 (g)
c)
H2 (g) + CuSO4 (aq)
→ H2SO4 (aq) + Cu (s)
7. Questions
1. Why is a glass rod used for stirring in this experiment, rather than a metal spatula ?
2. Explain the role of the zinc in this experiment.(look at the chemical equations above)
Why is the exact amount of zinc used not important? (where does it end up at the end of the lab?)
3. Explain the meaning of each of the following as they applied to the above experiment
a) a concentrated acid b) 20 - mesh granulated
c) effervescence
d) decant
e) supernatant
8. FOLLOW-UP problem:
1. An oxide of manganese is treated by the same process you used. Crystals of pure manganese are produced,
filtered, and air dried on your bench. Results are as follows;
mass of filter paper = 4.06 g
mass of dish = 104.27 g,
mass of fp +manganese (Fri)
= 5.21 g ,
mass of dish + oxide of manganese = 105.93 g,
mass of fp + manganese (Mon)
= 5.11 g
mass of fp + manganese (Tues)
= 5.11 g
Find the % manganese in the oxide and ** determine its formula.