Understanding Percent Composition by Mass

What is the basic formula for calculating percent composition by mass?

• Volume of element divided by total volume of compound

• Number of atoms of element divided by total number of atoms in compound

• Total mass of compound divided by molar mass of element

• Molar mass of element divided by total molar mass of compound

In the example of ethane (C2H6), why is the molar mass of carbon multiplied by 2?

• Because there are two carbon atoms in ethane

• To account for the atomic number of carbon

• To balance the equation

• To convert grams to moles

What is the percent composition by mass of carbon in ethane?

• 20.11%

• 79.89%

• 100%

• 50%

How can you find the percent composition of the final element in a compound?

• By adding the atomic masses of all elements

• By multiplying the atomic number by the molar mass

• By subtracting the known percent compositions from 100%

• By dividing the total mass by the number of elements

What is the percent composition by mass of hydrogen in ethane?

• 79.89%

• 50%

• 20.11%

• 100%