1. Aim # 6: How can we determine the percent composition (by mass) of a compound?
H.W. # 6
Study pp
Ans. ques. p. 189 # 28a,d; # 29a,c
Note: Calculate the % by mass of every element in the compounds.
Do Now: What volume will 8.5 g NH3(g) occupy at STP?

2. Given: an 88g sample of an unknown compound
Analysis shows that the sample contains g carbon
64g oxygen
What % of the total mass of this compound is from
carbon?
How can we calculate this value?

3. % carbon = mass carbon x 100 mass of sample
= 24g x = 27%
88g
% oxygen = mass of oxygen x 100
mass of sample
= 64g x = 73%

4. I percent composition – the relative amounts of each element in a compound. Their total must equal 100%.
the percentage of an element in a compound
% mass of element A = mass of element A x 100%
mass of compound
e.g for CO2
1. Assume you have 1.0 mole sample of your known
compound.

5. 2. Determine the mass of each element in the sample, and the molar mass of the sample.
1.0 mol C x 12.0g C = 12.0g C
1 mol C
2.0 mol O x 16.0g O = 32.0g O
1 mol O
mass of 1.0 mol CO2 = 12.0g g = 44.0 g

6. 3. Determine the percent (by mass) of each element in the compound.
% carbon in CO2 = g C x 100
44.0g CO2
= x = 27%
% oxygen in CO2 = g O x g CO2
= x = 73%

7. Now you try it. Calculate the % composition of KClO3 Answer:
Assume you have a 1.0 mole sample of KClO3
K: 1.0 mol K x g K = g K
1.0 mol K
Cl: 1.0 mol Cl x g C = g Cl
1.0 mol Cl
O: mol O x g O = g O
1.0 mol O

8. mass of 1.0 mol KClO3 = 39.0g + 35.5g + 48.0g = 122.5g KClO3
Next, determine the % by mass of each element in the compound.
% K in KClO3 = 39.0g x = 31.8%
122.5g
% Cl in KClO3 = 35.5g x = 29.0%
% O in KClO3 = g x = 39.2%

9. II Calculate the %, by mass, of the water of hydration in gypsum, CaSO4•2H2O
mass of 2H2O x
mass of CaSO4•2H2O
In one mole of compound,
mass of H2O = 2( ) = 36.0g H2O
mass of CaSO4•2H2O = 40.1g g + 4(16.0g)
+ 36.0g
= 172.2g

10. Calculate the % of water in BaCl2•2H2O
% H2O = g x 100% = 20.9% g
Calculate the % of water in BaCl2•2H2O