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# Ch. 6 - Chemical Quantities

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## Greg Evans

on 29 October 2013

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#### Transcript of Ch. 6 - Chemical Quantities

&
Ch. 6 - Chemical Quantities
4 sig figs
6.1 - Atomic Mass and Formula Mass
In most samples of elements we treat every atom as if it has the same atomic mass.
N= 14.01amu
Zn= 65.41amu
Sn= 118.7amu
14.01amu
1 N atom
OR
1 N atom
14.01amu
atomic mass conversion factor
Mass of a Sample of Atoms
atomic mass can be used to convert a known quantity of atoms to their mass (amu)
10 N atoms X
14.01amu
1 atom
=
140.1 amu
Using Atomic Mass to Count Atoms
6420amu X
1 S atom
32.07amu
=
2.00 X 10 S atoms
2
1500amu of N = ? atoms of N
Formula Mass
the sum of all the atomic masses of all the atoms in a formula
Formula Mass of Carbon Dioxide = ?
1 C atom X
12.01amu
1 C atom
=
12.01amu
2 O atoms X
16.00amu
1 O atom
=
32.00amu
Formula Mass of CO =
2
44.01amu
Formula Mass of water = ?
6.2 - The Mole
Just like eggs are counted by the dozen and paper is sold by the ream; atoms, molecules, and ions are counted in moles(mols).
12 eggs = 1 dozen
6.022 x 10 atoms = 1 mole
23
Subscripts State Moles of Elements
Subscripts in chemical formulas tell us how many atoms are in the compound.
If we have 1 mole of carbon dioxide how many moles of each element would we have? How many atoms?
1 mol of C and 2 mol of O
1 x 6.022 x 10 = atoms of C
23
2 x 6.022 x 10 = atoms of O
23
2 mol O
1 mol CO
2
1 mol C
1 mol CO
2
6.3 - Molar Mass
quantity in grams that equals the atomic mass of that element
6.4 - Calculations Using Molar Mass
molar mass is used to change moles into grams or grams into moles
equalities can be written as:
24.31 g Mg
1 mol Mg
1 mol Mg
24.31 g Mg
18.02 g H O
1 mol H O
2
2
2
2
&
1 mol H O
18.02 g H O
elements
compounds
? g Ag in a .750 mol sample
? mols of NaCl in a 737 g sample
pg. 176-177
g/mol
6.022 x 10
23
A 10.00 -lb bag to table sugar contains 4536 g sucrose, C H O . How many molecules of sugar are present?
or Atoms
element or compound
pg. 178
12
22
11
6.5 - Percent Composition and Empirical Formulas
Percent Composition - the mass percent of each element present in a compound
Molecular Formula - actual or true formulas of compounds
Empirical Formula - formula that represents the lowest whole-number ratio of the atoms in a compound
Empirical Formulas
determined by converting the # of grams of each element to moles and finding the lowest whole # ratio to use as subscripts
Converting Decimals to Whole numbers
Sometimes the result of dividing by the smallest number of moles gives a decimal instead of a whole number.
less than 0.1 or greater than 0.9, round to the nearest whole number
between 0.1 & 0.9, multiply by small integer until a whole number is obtained
Try the sample on pg. 184
6.6 Molecular Formulas
Molecular formula = small integer x empirical formula
Molar mass = small integer x empirical formula mass
small integer =
molar mass of compound (g/mol)
empirical formula mass (g/mol)
molar mass of ribose
empirical formula mass of ribose
=
150.0 g/mol
30.0 g/mol
= 5
5 x empirical formula (CH O) = molecular formula (C H O )
2
5
10
5
Calculating Molecular
Formula
Calculate mass of empirical formula
Divide molar mass by the empirical formula mass to obtain a small integer
Multiply the empirical formula by the small integer to obtain molecular formula
Try sample problem on pg. 187
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