General Chemistry 1

What formula calculates the percent composition of an element in a compound?

• (Mass of element / Molar mass of compound) × 100

• (Molar mass of compound / Mass of element) × 100

• (Number of atoms / Total atoms) × 100

• (Mass of element / Total mass of elements) × 100

Why do the percent compositions of all elements in a compound sum to 100%?

• It's a standard rule in chemistry

• The compound is always pure

• All element masses equal the compound's molar mass

• Percentages are molar mass-based

What is the percent composition of oxygen in H₂O (molar mass 18 g/mol)?

• 88.9%

• 11.1%

• 50%

• 25%

A compound is 75% carbon and 25% hydrogen by mass. What is the mass of carbon in 80 g of the compound?

• 25 g

• 80 g

• 60 g

• 20 g

If two compounds share the same percent composition, must they have the same molecular formula?

• Yes, without exception

• No, it depends on their molar masses

• Yes, if they are ionic

• Yes, if their structures are identical

How do empirical and molecular formulas differ?

• Empirical is the simplest ratio; molecular is the actual number of atoms

• Empirical is for gases; molecular is for solids

• Empirical shows exact counts; molecular shows ratios

• Molecular is always simpler than empirical

What information is needed to find a molecular formula from an empirical formula?

• Percent composition

• The compound's molar mass

• Atomic masses only

• The number of atoms

A compound's empirical formula is CH, and its molar mass is 78 g/mol. What is its molecular formula? (CH molar mass = 13 g/mol)

• CH

• C₃H₃

• C₆H₆

• C₂H₂

A compound is 52.2% carbon, 13.0% hydrogen, and 34.8% oxygen. What is its empirical formula?

• C₂H₅O

• CH₂O

• C₃H₈O₂

• CHO

Which step is unnecessary when finding an empirical formula from percent composition?

• Convert masses to moles

• Assume a 100 g sample

• Calculate the compound's molar mass

• Determine the simplest mole ratio