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Atomic Masses

A. Average mass is found by taking the average of all of an element’s isotopes masses,

taking into account the percentage of which appear naturally on earth

a. Ex: Most of the carbon found naturally is carbon-12 (98.8%) and a small

percentage is carbon-13 (1.1%). Carbon-14 has a negligible percentage. So to find

the average mass you do

i. (.0988 x 12amu) + (.0111 x 13.003355 amu)= 12.01 amu

The Mole

A. Used in counting atoms

B. Mole: the number equal to the number of carbon atoms in exactly 12 grams of pure

carbon-12

C. Avogadro’s Number: One mole of something consists of 6.022 x 10^23 units of the

substance

a. 6.022 x 10^23 atoms in one mole

D. A sample of a natural element with a mass equal to the element’s atomic mass expressed

in grams contains 1 mole of atoms

E. 1 mole= 1 gram

Molar Mass

A. The mass in grams of 1 mole of a compound

Percent Composition of Compounds

A. Mass Percent: found by comparing the mass of one element of a compound to the total

mass of the compound and multiplying by 100%

Determining the Formula of a Compound

A. Empirical Formula: the simplest whole-number ratio of the various types of atoms in a

compound

B. Determining an empirical formula

a. Base calculation on 100 grams. Each percent will represent the mass in grams of

that element in the compound

b. Determine number of moles of each element present in 100 grams using the

atomic weights

c. Divide each value of the number of moles by the smallest of the values. The

results should be a whole number that represents the subscripts of the elements

d. If they are not whole numbers, multiply each number by an integer

C. Determining molecular formula

a. Method 1

i. Obtain the empirical formula

ii. Compute the empirical formula mass

iii. Calculate the ratio of molar mass: empirical formula mass

iv. Multiply the resulting integer by the subscripts

b. Method 2

i. Use the mass percents and the molar mass to determine the mass of each

element present in 1 mole of a compound

ii. Determine the number of moles of each element present in 1 mole of the

compound

iii. Integers from the previous steps represent the subscripts in formula

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