Chemical QuantitiesChapter 7 (10)

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