chemical formulas and compounds using chemical formulas

Chemical Formulas and Compounds

Using Chemical Formulas


Objective

• Calculate the formula mass or molar mass of any given compound

• Use molar mass to convert between mass in grams and amount in moles of a chemical compound

• Calculate the number of molecules, formula units, or ions in a given molar amount of a chemical compound

• Calculate the percentage composition of a given chemical compound


Chemical Formulas

• A chemical formula indicates

• the elements present in a compound

• the relative number of atoms or ions of each element present in a compound

• Chemical formulas also allow chemists to calculate a number of other characteristic values for a compound:

• formula mass

• molar mass

• percentage composition


Formula Masses

• The formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all atoms represented in its formula

formula mass of water, H2O average atomic mass of H: 1.01 amuaverage atomic mass of O: 16.00 amu

1.01 amu2 H atoms 2.02 amu

H atom

16.00 amu1 O atom 16.00 amu

O atom

average mass of H2O molecule = 18.02 amu


Formula Masses

• The mass of a water molecule can be referred to as a molecular mass

• The mass of one formula unit of an ionic compound, such as NaCl, is not a molecular mass

• The mass of any unit represented by a chemical formula (H2O, NaCl) can be referred to as the formula mass


Formula Masses

• Sample Problem

• Find the formula mass of potassium chlorate, KClO3


Formula Masses

• Sample Problem Solution

• The mass of a formula unit of KClO3 is found by adding the masses of one K atom, one Cl atom, and three O atoms

39.10 amu1 K atom 39.10 amu

K atom

35.45 amu1 Cl atom 35.45 amu

Cl atom

16.00 amu3 O atoms 48.00 amu

O atom

formula mass of KClO3 = 122.55 amu


Molar Masses

• The molar mass of a substance is equal to the mass in grams of one mole, or approximately 6.022 × 1023 particles, of the substance

• The molar mass of a compound is calculated by adding the masses of the elements present in a mole of the molecules or formula units that make up the compound


Molar Masses

• One mole of water molecules contains exactly two moles of H atoms and one mole of O atoms. The molar mass of water is calculated as follows

• A compound’s molar mass is numerically equal to its formula mass

1.01 g H2 mol H 2.02 g H

mol H

16.00 g O1 mol O 16.00 g O

mol O

molar mass of H2O molecule = 18.02 g/mol


Formula Masses

• Sample Problem

• What is the molar mass of barium nitrate, Ba(NO3)2

Formula Masses


• One mole of barium nitrate, contains one mole of Ba, two moles of N (1 × 2), and six moles of O (3 × 2)

137.33 g H1 mol Ba 137.33 g Ba

mol Ba

16.00 g O6 mol O 96.00 g O

mol O

14.01 g2 mol N 28.02 g N

mol N


molar mass of Ba(NO3)2 = 261.35 g/mol


Molar Mass Conversions



• Sample Problem

• What is the mass in grams of 2.50 mol of oxygen gas (O2)



• Given: 2.50 mol O2

• Unknown: mass of O2 in grams

2

22

2

32.00 g O2.50 m 80.0ol O

mol Og O


moles O2 grams O2

amount of O2 (mol) × molar mass of O2 (g/mol) = mass of O2 (g)

16.00 g O2 mol O 32.00 g

mol O



• Sample Problem

• Ibuprofen, C13H18O2, is the active ingredient in many nonprescription pain relievers. Its molar mass is 206.31 g/mol

• If the tablets in a bottle contain a total of 33g of ibuprofen, how many moles of ibuprofen are in the bottle?

• How many molecules of ibuprofen are in the bottle?

• What is the total mass in grams of carbon in 33g of ibuprofen?



• Given: 33g of C13H18O2, molar mass 206.31 g/mol

• Unknown: moles C13H18O2

molecules C13H18O2

total mass of C




• Sample Problem Solutiongrams moles

13 18 2

13 18 2 13 18 213 18 2

1 mol C H Og C H O mol C H O

206.31 g C H O

moles molecules

23

13 18 2 13 18 2

6.022 10 moleculesmol C H O molecules C H O

mol

moles C13H18O2 moles C grams C

13 18 213 18 2

13 mol C 12.01 g Cmol C H O g C

mol C H O mol C




23

2

13 18

2

2

13 18 2

6.022 10 molecules0.16mol C

9.6 10 molecules C

H Omol

H O

13 18 2

13 18 2

13 mol C 12.01 g C0.16 mol C H O

mol C H O mol C25 g C

13 18 2

13 18 213 18 2

13 18 2

1 mol C H O33 g C H O

206.31 g C H O0.16 mol C H O


Percentage Composition

• It is often useful to know the percentage by mass of a particular element in a chemical compound

• To find the mass percentage of an element in a compound, the following equation can be used

• The mass percentage of an element in a compound is the same regardless of the sample’s size

mass of element in sample of compound100

mass of sample of compound

% element in compound



• The percentage of an element in a compound can be calculated by determining how many grams of the element are present in one mole of the compound

• The percentage by mass of each element in a compound is known as the percentage composition of the compound

mass of element in 1 mol of compound100

molar mass of compound

% element in compound



• Sample Problem

• Find the percentage composition of copper(I) sulfide, Cu2S



• Given: formula, Cu2S

• Unknown: percentage composition of Cu2S


formula molar mass mass percentage of each element




63.55 g Cu2 mol Cu 127.1 g Cu

mol Cu

32.07 g S1 mol S 32.07 g S

mol S

Molar mass of Cu2S = 159.2 g

2

127.1 g Cu100

159.2 g C79.8 u

S5% C

u

2

32.07 g S100

159.2 g C20. S

u S15%



• Sample Problem

• As some salts crystallize from a water solution, they bind water molecules in their crystal structure. Sodium carbonate forms such a hydrate, in which 10 water molecules are present for every formula unit of sodium carbonate. Find the mass percentage of water in sodium carbonate decahydrate, Na2CO3•10H2O, which has a molar mass of 286.19 g/mol.

chemical formulas and compounds using chemical formulas

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