Empirical Formulas and Percent Composition

The video tutorial covers percent composition and empirical formulas. It revisits percent composition by calculating the percent by mass of oxygen in water and nitric acid. The tutorial then explains empirical formulas, which represent the lowest ratio of moles of each element in a compound. Examples include determining empirical formulas for compounds with given percent compositions, such as a compound with carbon and chlorine, and propane, a hydrocarbon. The tutorial emphasizes converting percentages to grams, dividing by molar mass to find moles, and adjusting ratios to whole numbers.

What is the percent by mass of oxygen in a water molecule?

• 76.2%

• 50%

• 33.3%

• 88.9%

How do you calculate the percent composition of an element in a compound?

• Multiply the mass of the element by the total mass of the compound

• Divide the mass of the element by the total mass of the compound and multiply by 100

• Subtract the mass of the element from the total mass of the compound

• Add the mass of the element to the total mass of the compound

What is the percent by mass of oxygen in nitric acid?

• 76.2%

• 88.9%

• 33.3%

• 50%

What is an empirical formula?

• The exact number of atoms of each element in a compound

• The lowest whole number ratio of moles of each element in a compound

• The percent composition of each element in a compound

• The total mass of a compound

How do you determine the empirical formula from percent composition?

• Convert percentages to grams, divide by molar mass, and find the lowest whole number ratio

• Add the percentages together and divide by the number of elements

• Subtract the smallest percentage from the largest

• Multiply the percentages by the molar mass

In the example with carbon and chlorine, what is the empirical formula determined?

• C2Cl2

• C1Cl4

• C4Cl1

• C3Cl3

What is the empirical formula of propane based on the given percentages?

• C3H8

• C2H6

• CH4

• C4H10

Why can't the number 2.67 be rounded to 3 in the propane example?

• Because it is less than 2.1

• Because it is not greater than 2.9

• Because it is exactly 2.5

• Because it is already a whole number

What adjustment is made to the moles of hydrogen in the propane example?

• Subtract 1

• Add 1

• Divide by 3

• Multiply by 3

What is the final ratio of moles of carbon to hydrogen in propane?

• 2:6

• 3:8

• 4:10

• 1:4