Composition

Question 1

Law of Definite Proportions

Answer 1

A statement that a compound always contains the same proportion of elements by mass (same %).

Grams = %

Ex.

4 g of Hydrogen = 4% of Hydrogen

Question 2

Percentage Composition

Answer 2

• The percentage by mass of each element in a compound.
• The mass of a singular element in a compound divided by the total mass of the compound.
• Determinable because of the Law of Definite Proportions.

Question 3

Calculating Percentage Composition

Answer 3

Can be determined with the following formula:

%Element = m element / m compound x 100%

• Note: Both masses should have the same unit. The answer has NO units because it’s a %.

Ex. A 27.0g sample of an unknown compound, CxHyOz contains 7200.0mg of carbon, and 2200.0mg of hydrogen. The remaining mass is oxygen. Determine the percentage composition of this unknown:

mCxHyOz = 27.0g 
mC = 7.2g
mH = 2.2g
mO = 17.6g

Mass%C = 7.2/27 x100% = 26.7%
Mass%H = 2.2/27 x100% = 8.15%
Mass%O = 17.65 x100% = 65.2%

Question 4

Percentage Composition of Alloys and Mixtures

Answer 4

• An alloy is a mixture of different metals that form useful products.
• Stainless stew used for cutlery contains 10% chromium by mass.
• Stainless steel used for surgical tools contain 18% chromium by mass making it more resistant to corrosion.