In the lab I ended up with 0.44 grams of MgCl2.
How much of that was magnesium and how much
chlorine?
25.5% Magnesium
74.5 % Chlorine
This is the same for everyone and every MgCl2
compound!
Percent Composition
 The
percent composition is the mass
percentage of each type of atom
(element) in a compound.
%
by mass =
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑝𝑎𝑟𝑡
𝑚𝑎𝑠𝑠 𝑜𝑓 𝑤ℎ𝑜𝑙𝑒
x 100
Percent Composition from
Chemical Formula
To calculate the percent composition of a component in a
compound:
1.
Find the molar mass of the compound by adding up the
masses of each atom in the compound.
2.
Calculate the molar mass of the component in the
compound for which you are solving.
3.
Divide the molar mass of the component by the total
molar mass of the compound and multiply by 100
 Calculate
the percent by mass of
nitrogen in caffeine (C8H10N4O2)
 Step1 – Find the Molar Mass
8C + 10H +4N + 2O
(8 x 12.0 g/mol)
+ (10 x 1.0 g/mol)
+ ( 4 x 14.0 g/mol)
+ (2 x 16.0 g/mol)
= 194.0 g/mol
Calculate
the percent by mass of
nitrogen in caffeine (C8H10N4O2
Step
2 – Divide part by Whole
Nitrogen:
4 𝑥 14.0 𝑔/𝑚𝑜𝑙
194 𝑔/𝑚𝑜𝑙
x 100 = 28.9 %
PERCENT COMPOSITION BY MASS OF CAFFEINE
Nitrogen
Carbon
Oxygen
5.1%
16.5%
49.5%
28.9%
Hydrogen