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What is Percent Composition?
100Whole
PartPercent
100samplewholeofmass
elementofmassncompositioPercent
Example #1
Example:A 8.5 g sample is composed of carbon and
hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of
the sample?
Example #1
If total = 8.5 g
and C = 5.5 g
then H = 3.0 g
-
1005.8
5.5%
g
gC 100
5.8
0.3%
g
gH
%7.64% C %3.35% H
Example:A 8.5 g sample is composed of carbon and
hydrogen. If 5.5 g of the sample is found to be carbon, what is the percent composition by mass of
the sample?
Let’s Practice #1
A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?
Let’s Practice #1
1005.27
?%0.57
g
gCl
gClg
?%0.57100
5.27
27.5 g 27.5 g
100 100
gClg ?7.15
A sample is 57.0% by mass chlorine, how many grams of chlorine are present in a 27.5 g sample?
Example #2
Percent composition can also be determined from a chemical formula
Example:Find the percent composition, by mass, of CaCl2
Example #2
Percent composition can also be determined from a chemical formula
total = 110.98 g
Ca = 40.08 g
Cl = 70.90 g
10098.110
08.40%
g
gCa 100
98.110
90.70%
g
gCl
%1.36% Ca %9.63% Cl
CaCl
12
40.08 g/mole35.45 g/mole
= 40.08 g/mole= 70.90 g/mole +
110.98 g/mole
So for 1 mole:
Example:Find the percent composition, by mass, of CaCl2
Let’s Practice #2
10000.85
99.22%
g
gNa 100
00.85
01.14%
g
gN
%0.27% Na %5.16% N
Find the percent composition, by mass, of NaNO3
NaN
11
22.99 g/mole14.01 g/mole
= 22.99 g/mole= 14.01 g/mole
+
85.00 g/mole
O 3 16.00 g/mole = 48.00 g/mole
10000.85
00.48%
g
gO
%5.56% O
What’s an Empirical Formula?
Empirical – from data
Empirical Formula – Chemical formula determined from lab data. Lowest possible ratio of atoms
CH2 is the lowest ratio (and empirical formula) of the molecule C3H6
Ratio of Atoms in a Molecule
Subscripts in a chemical formula show the ratio of atoms (or ions) in a molecule
a sample of CaCl2 has 1 calcium ion : 2 chlorine ions
We can use the unit “mole” to count things
Atoms and ions can be counted by “moles”
If the subscripts give the ratio of atoms, then they also give the ratio of moles of atoms
a sample of CaCl2 has 1 mole of calcium ions : 2 moles of chlorine ions
Bmoles
Amoles
Batoms
Aatoms
#
#
Using Mole Ratio of Atoms in a Molecule
Therefore, if the ratio of moles of each atom is found…
then the subscripts of the chemical formula are known
1 mole C2 mole H
CH2
Example #3If given percents, use those percents as grams (as if you assume you have a 100 g sample)—Remember %’s add up to 100!
Change grams to moles for each atom
Find the lowest possible whole number ratio of the atom (divide all moles by the smallest # of moles)
Use the ratio as subscripts for writing the chemical formula
1
2
3
4
Example:Find the empirical formula if a sample contains Ca
and Cl and is 36.1% Ca
Example #3If given percents, use those percents as grams (as if you assume you have a 100 g sample)—Remember %’s add up to 100!
Change grams to moles for each atom
Find the lowest possible whole number ratio of the atom (divide all moles by the smallest # of moles)
Use the ratio as subscripts for writing the chemical formula
1
2
3
4
36.1 g Ca= _____ mol Cag Ca
mol Ca 140.08
0.901
63.9 g Cl= _____ mol Clg Cl
mol Cl 135.45
1.80
0.901 mol Ca = 1 mol Ca 0.901
1.80 mol Cl = 2 mol Cl 0.901
CaCl2
Example:Find the empirical formula if a sample contains Ca
and Cl and is 36.1% Ca
Let’s Practice #3
40.92 g C= _____ mol Cg C
mol C 112.01
3.41
4.58 g H= _____ mol Hg H
mol H 11.01
4.53
3.41 mol C = 1 mol C 3.41
4.53 mol H = 1.33 mol H 3.41
C3H4O3
54.5 g O= _____ mol Og O
mol O 116.00
3.41
3.41 mol O = 1 mol O 3.41
Multiply the ratio (1 : 1.33 : 1) by 3to make a whole number ratio (3 : 4: 3)
Find the empirical formula if a sample contains
40.92 g C, 4.58 g H and 54.5 g O
What’s a Molecular Formula?
Empirical Formula – Chemical formula determined from lab data. Lowest possible ratio of atoms
Molecular Formula – Actual ratio of atoms in a molecule
Empirical versus Molecular Formula
The empirical formula is the lowest possible ratio.
A molecule with the empirical formula:
Could have one of the following molecular
formulas:
NO2 NO2, N2O4, N4O8…
CH2 CH2, C2H4, C4H8…
The molecular formula is the actual ratio
Example #4
Find the empirical formula, if not given
Find the molar mass of the empirical formula
Find the ratio of the molecular formula’s molar mass (must be given to you) to the empirical formula’s molar mass
Multiply the empirical formula’s subscripts by the ratio found in step 3.
1
2
3
4
Example:Empirical formula = C3H4O3. The molecular
formula’s molar mass = 176.14 g/mole.Find the molecular formula.
Example #4
Find the empirical formula, if not given
Find the molar mass of the empirical formula
Find the ratio of the molecular formula’s molar mass (must be given to you) to the empirical formula’s molar mass
Multiply the empirical formula’s subscripts by the ratio found in step 3.
1
2
3
4
C3H4O3
176.14 g/mole = 2 88.07 g/mole
3
3
12.01 g/mole
16.00 g/mole
= 36.03 g/mole
= 48.00 g/mole+
88.07 g/mole
4 1.01 g/mole = 4.04 g/moleC
OH 2
C6H8O6
Example:Empirical formula = C3H4O3. The molecular
formula’s molar mass = 176.14 g/mole.Find the molecular formula.
What’s a Hydrate?
Hydrate – Molecule that has water physically attached to it
It’s not dissolved in water…hydrates can be solid, liquid or gas!
Hydrates are inorganic salts "containing water molecules combined in a definite ratio as an integral part of the crystal“
either bound to a metal center or have crystallized with the metal
When writing a hydrate, a dot is used to separate the formula compound from the number of water moleculesExamples)
magnesium sulfate heptahydrate
cobalt (II) chloride hexahydrate
tin (II) chloride dihydrate
Hydrate & anhydride
Hydrate = molecule with water molecules physically attached
anhydride = molecule with water removedThe water can be removed by heating the
hydrate
Hydrate anhydride + waterheat
Finding the ratio of anhydride molecules to water molecules gives you the hydrate formula
Example #5
Find the mass of anhydride & water if not given
Change mass of anhydride & water to moles
Find the ratio of the moles water to moles anhydride
Write the hydrate formula
1
2
3
4
Example:2.46 g MgSO4 hydrate is heated and 1.20 g MgSO4 anhydride is left. Find the hydrate formula.
Example #5
Find the mass of anhydride & water if not given
Change mass of anhydride & water to moles
Find the ratio of the moles water to moles anhydride
Write the hydrate formula
1
2
3
4
Example:2.46 g MgSO4 hydrate is heated and 1.20 g MgSO4 anhydride is left. Find the hydrate formula.
1.20 g MgSO4 = _______ mol MgSO4g MgSO4
mol MgSO4 1120.38
0.00997
1.26 g H2O = _______ mol H2Og H2Omol H2O 1
18.020.0699
0.0699 mol H2O = 7.01 0.00997 mole MgSO4
MgSO4 7 H2O
Hydrate = anhydride + water2.46 g = 1.20 g + water
Water = 1.26 g