chemical quantities chapter 7 (10). how many dozen objects are present when you have: a) 60...
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How many dozen objects are present when you have:
a) 60 objects: 5 dozen
b) 3 objects: 1/4 dozen or 0.25 dozen
c) 41 objects: 3.4 dozen
d) 2.5 objects: 0.21 dozen
What did you use as the basis for the conversions?
1 dozen objects = 12 objects
12 objects = 1 dozen
60 objects x 12 objects1 dozen = 5 dozen
2.5 objects x 12 objects1 dozen = .21 dozen
How many objects are present when you have:
a) 6.0 dozen: 72 objects
b) 3.2 dozen: 38.4 objects
c) 0.025 dozen: 0.30 objects
d) 3.25 x 10-3 dozen: 0.0390 objects
What did you use as the basis for the conversions?
1 dozen objects = 12 objects12 objects = 1 dozen
6.0 dozen x 12 objects
1 dozen
= 72 objects
0.025 dozen x = 0.30 objects12 objects
1 dozen
Solve these problems:
a) 1.3 x 102 dozen: 1.56 x 103
b) 5.5 objects: 0.458
c) 2.5 x 10-4 objects: 2.08 x 10-5
d) 7.4 x 10-1 dozen: 8.88
objects
dozen
dozen
objects
If each object weighs 10 grams, how many objects do you have when you have 90 grams worth?
90 g x 10 g
1 object = 9 objects
If each object weighs 10 grams, how many grams of objects do you have when you have 3.0 dozen objects?
3.0 dozen 1 dozen
12 objects = 360 g 1 object
10 g
Solve these problems:
a) How many dozen objects are present when you have 100. g of objects? Each dozen weighs 32 g.
100. g32 g
1 dozen= 3.13 dozen
Solve these problems:
b) How many objects are present when you have 100. g of objects? Each dozen weighs 32 g.
100. g32 g
1 dozen= 37.5 objects
1 dozen12 objects
Solve these problems:
c) How much do 1.25 dozen objects weigh if each dozen weighs 32 g?
1.25 doz 32 g
1 dozen= 40.0 grams
I. The Mole ConceptA. Just as “1 dozen = 12”, chemistry uses
this ratio:
1 mole = 6.02 x 1023
B. This number is frequently referred to as:
Avogadro’s Number
1) How many atoms of carbon are present in 0.250 moles of carbon?
0.250 mol 6.02 x 1023 atoms
1 mole
= 1.51 x 1023 atoms
2) How many moles of carbon atoms are present in 5.00 x 1010 atoms of carbon?
5.00 x 1010 atoms
6.02 x 1023 atoms
1 mole
= 8.31 x 10-14 mol
3) The weight of 1 mole of an element (6.02 x 1023 atoms of that element) = it’s atomic weight
0.330 mole C
1 mole= 3.96 g
a) What is the mass of 0.330 mol of carbon?
12.011 g
100. g 1 mole= 8.33 mole
b) How many moles are present in 1.00 x 102 grams of carbon?
12.011 g
c) How many atoms of carbon are present in 2.00 grams of carbon?
2.00 g 6.02 x 1023 atoms
12.011 g
1 mole
1 mole= 1.00 e23
3.0 x 109 atoms 1 mole
d) How much do 3.0 x 109 atoms of carbon weigh?
12.011 g
= 6.0 x 10-14 g
6.02 x 1023 atoms 1 mole
0.125 mol H2O
1 mole H2O
e) How many grams do 0.125 moles of H2O weigh?
18 g = 2.25 g
90. g H2O 1 mole H2O
f) How many molecules of water are present in 90. grams of water?
18 g
6.02 x 1023 molec
1 mole H2O
= 3.0 x 1024 molecules
II. Percent CompositionA. Percent = Part x 100
whole
Ex: What is the percent boys in a class made up of 5 boys and 15 girls?
% boys = 5 boys x 100
20 total = 25% boys
Ex: What is the percent oxygen in H2O, by weight?
% oxygen = 16 g oxygen x 100
18 g total
= 88.9% oxygen
III. Empirical and Molecular Formulas
A. The subscripts in a chemical formula are the mole ratios of atoms
Ex: H2O has 2 moles of hydrogen : 1 mole of oxygen in every 1 mole of water
Ex: C6H12O6 has 6 moles of carbon : 12 mole of hydrogen: 6 moles of oxygen in every 1 mole of C6H12O6
B. If the percent composition is known, the molar ratio of each element can be calculated:
Ex: A sample is known to consist of 88.9% O and 11.1% H. What is the formula of this compound?
88.9 O
11.1 H
Div by a.w.
Div by smallest
5.56
11.1
1
2
H2O
Ex: A sample is known to consist of 40.0% C and 6.70% H and the rest is oxygen. What is the empirical formula of this compound? If the molecular weight is 180 g, what is the molecular formula?
40.0 C
6.70 H
53.3 O
Div by a.w.
Div by smallest
3.30
6.70
1
2
CH2O
3.30 1
Empirical Formula
(Smallest whole number ratio)
Molecular Formula = C6H12O6