SCH3U
Percentage Composition of Magnesium Chloride
Name:
Introduction:
The composition of a compound is often expressed in terms of its percentage composition by mass. This means the
percentage of the total mass of the compound contributed by each element. The percentage composition of a
compound can be determined using two methods.
i) Predicted (expected) Percentage Composition: Is calculated using the atomic masses from the periodic table of
each of the respective elements and their ratio according to the compounds formula.
ii) Experimental (observed) Percentage Composition: Is calculated using data obtained during a chemical
reaction.
Purpose:
To complete a single displacement reaction between magnesium and hydrochloric acid and obtain results to
calculate the experimental percentage composition. From the periodic table, calculate the predicted percentage
calculation. In an effort to compare the two methods, complete an error analysis which includes sources of error and
a percentage error calculation.
Materials:
Equipment:
Chemicals :
safety goggles,scissors, evaporating dish, eyedropper, hot plate
magnesium ribbon (0.3 g), hydrochloric acid (6.0 mol/L)
Procedure:
1.
Obtain a magnesium strip approximately 15 cm in length.
2a.
Use steel wool and clean any oxide coating off the magnesium strip.
2b.
Mass the magnesium strip. Record your mass in the observation chart provided.
3a.
Clean and dry an evaporating dish.
3b.
Mass the evaporating dish. Record your mass in the observation chart provided.
4.
Carefully cut your magnesium strip into 1 cm length pieces and place these into your evaporating dish.
5.
Drop by drop, carefully add the concentrated hydrochloric acid to the magnesium strips until the magnesium
has completely reacted.
6.
Place the evaporating dish on the hot plate and heat gently until all liquid has been evaporated.
7.
Mass the evaporating dish and product. Record your mass in the observation chart provided.
Pre-lab Calculations / Questions:
1.
Write a balanced chemical equation to illustrate the single displacement reaction that occurred.
2.
Predicted (expected) Percentage Composition Chart: show all work in chart form (as suggested below).
Molar mass of magnesium chloride
% magnesium in magnesium chloride
% chlorine in magnesium chloride
Observations:
A. Quantitative Observation Chart
Mass of magnesium strip (g)
Mass of evaporating dish (g)
Mass of evaporating dish and product (g)
B. Experimental (observed) Percentage Composition: complete all work in chart form (as suggested below).
Mass of product
Mass of magnesium reacted into product
Mass of chlorine in product
% magnesium in magnesium chloride
% chlorine in magnesium chloride
Discussion Questions :
1. List 2 sources of error.
2. Calculate the percent error in composition using the formula found below:
| experimental result – expected result| X 100
Expected result
**Complete all questions on a separate sheet of paper. Show all of your work (pay attention to set up, units
and significant figures).
Conclusion: