Question 1

What is the law of constant composition?

Accepted Answer

In any chemical compound, elements always bind in the same proportion with each other

(water is always made of hydrogen and oxygen in a 2:1 ratio)

Question 2

What is the percent composition of a substance?

Accepted Answer

The percent by mass of each element in a compound

(water is 11.11% hydrogen by mass and 88.89% oxygen)

Question 3

What is a compound's empirical formula?

Accepted Answer

The simplest whole-number ratio of atoms in a compound

( and  both have the empirical formula of )

Question 4

What is a compound's molecular formula?

Accepted Answer

A representation of that chemical symbols to indicate elements, and subscripts to indicate the number of atoms

( is the molecular formula for glucose; the lowest ratio of the molecular formula is the empirical formula)

Question 5

How do you find a substance's percent composition?

Accepted Answer

Find the mass of each element in the compound, and divide it by the total mass of the compound

Question 6

How do you find a compound's empirical formula?

Accepted Answer

Start: Assume a 100g sample

Find # of moles: Convert each element from grams into moles, using its atomic mass

Whole number ratio: Divide all the moles by whichever is smallest

Write formula: If all moles are whole numbers, write the moles as the subscript for each element

(if the moles are not whole numbers, you might need to multiply everything by a number)

Question 7

How do you find a compound's molecular formula?

Accepted Answer

Find its empirical formula

Find its atomic mass

Scale the number of atoms per element to match the actual atomic mass to the given atomic mass

Elemental Composition of Pure Substances, AP Chemistry Flashcards