Honors Chemistry Section 7.3. A chemical formula indicates: ◦the elements present in a compound...
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Transcript of Honors Chemistry Section 7.3. A chemical formula indicates: ◦the elements present in a compound...
![Page 1: Honors Chemistry Section 7.3. A chemical formula indicates: ◦the elements present in a compound ◦the relative number of atoms or ions of each element.](https://reader036.fdocuments.us/reader036/viewer/2022062715/56649d955503460f94a7d718/html5/thumbnails/1.jpg)
Honors ChemistrySection 7.3
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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
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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.◦ example: formula mass of water, H2O
average atomic mass of H: 1.01 amuaverage atomic mass of O: 16.00
amusFormulaMas
amuOatom
amuOatom
amuHatom
amuHatoms
02.18
00.161
00.161
02.21
01.12
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Problem
Find the formula mass of Na2SO3
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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.
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Formula Masses
Video
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Problem
Find the formula mass of potassium chlorate, HClO3.
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The Mole
Video
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Molar Masses
The molar mass of a substance is equal to the mass in grams of one mole, or 6.022 × 1023 particles, of the substance.◦ example: the molar mass of pure calcium, Ca, is 40.08 g/mol
because one mole of calcium atoms has a mass of 40.08 g.
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.
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Calculating Molar Masses for Ionic Compounds
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Problem
What is the molar mass of barium nitrate, Ba(NO3)2?
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Problem
How many moles of each atom are in one mole of K2SO4?
How many moles of each atom are in one mole of (NH4)2CrO4?
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Molar Mass as a Conversion Factor
The molar mass of a compound can be used as a conversion factor to relate an amount in moles to a mass in grams for a given substance.
To convert moles to grams, multiply the amount in moles by the molar mass:
Amount in moles × molar mass (g/mol) = mass in grams
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Calculations
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Molar Mass as a Conversion Factor
Video
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Problem
Calculate the molar mass of O2.
Use the molar mass of O2 to convert 2.50 moles to mass
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Problem
What is the mass of 3.04 moles of NH3?
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Problem
How many moles in 4.15 x 10-3 g of C6H12O6?
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Problem
How many molecules are in this mass of C6H12O6?
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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.
positionPercentComoundMassOfComp
entMassOfElem100
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Percentage Composition
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.
positionPercentComfCompoundMolarMassO
oundMoleOfCompentInMassOfElem100
1
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Water of Hydration
Some compounds have water molecules as part of their crystal structure. These substances are call hydrates.◦Ex. – Na2SO4 ・ 10H2O
This water is included when calculating the molar mass.
You can drive this water off by careful heating.
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Problem
What is the percentage of water in CaSO4·2H2O?
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Problem
NH4NO3 is 35.0% N. How much N is in 49.0g of the compound?
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Percentage Composition
Video
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Percentage Composition Calculations
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Problem
Find the percentage composition of Ag2SO4.