Atoms & Moles; Molar Mass & % Composition; … 6 Atoms & Moles; Molar Mass & % Composition; Formulas of Compounds A. Counting by Weighing • We can count objects by knowing how much

Chapter 6 Atoms & Moles; Molar Mass & % Composition; Formulas of Compounds

1.  To understand the concept of average mass 2.  To learn how counting can be done by weighing 3.  To understand atomic mass and learn how it is

determined 4.  To understand the mole concept and Avogadro’s

number 5.  To learn to convert among moles, mass, and number of

atoms

Objectives


Some Questions……… •  How many atoms are there in a bucket of water? How

could we determine that without having to count them all individually?

•  I know that one atom of carbon reacts with one molecule of oxygen to make one molecule of carbon dioxide. If I have a sample of carbon how can I predict how much oxygen to react with it? Can I do it by counting out atoms? C + O2 reacts to give CO2

•  What do the atomic masses in the Periodic Table stand for? periodic table


A. Counting by Weighing

•  We can count objects by knowing how much each one weighs and weighing the sample

•  Objects do not need to have identical masses to be counted by weighing. –  All we need to know is the average mass of the

objects. •  To count the atoms in a sample of a given element by

weighing we must know the mass of the sample and the average mass for that element.

=


A. Counting by Weighing

Averaging the Mass of Similar Objects

Example: How can we count out 1000 jelly beans?

3.  Now we can find the average mass of a bean.

4.  Finally we can multiply to find the mass of 1000 beans. We can then weigh out 1000 beans.

1.  Not all jelly beans have the same mass. 2.  Suppose we weigh 10 jelly beans and find:


B. Atomic Masses: How Much Do Atoms Weigh?

•  Atoms have very tiny masses so scientists made a unit to avoid using very small numbers.

•  1 atomic mass unit (amu) is 1/12 of the mass of a 12C atom

•  1 amu = 1.66 ×10-24 g

How many amu’s does a 12C atom weigh? How many grams does a 12C atom weigh?


B. Atomic Masses: Counting Atoms by Weighing

•  The average atomic mass for an element is the weighted average of the masses of all the isotopes of an element.

•  The Periodic Table shows the atomic mass of each element in amu’s. This is different to the mass number

periodic table What is the average atomic mass in amu’s of an atom of:

Barium

Fluorine

Xenon

Nickel (P210 Q5,6)


Isotope Atomic Mass amu

Natural Abundance %

Abundance x Atomic Mass

54Fe 53.9396127 5.845 3.153

56Fe 55.9349393 91.754 51.323

57Fe 56.9353958 2.119 1.206

58Fe 57.9332773 0.282 0.163

Calculate the Average Atomic Mass of Naturally Occurring Iron:

Sum of the final column = 55.845 amu


Calculate the atomic masses of one of the following elements. Use the isotope masses and % abundance from the Dynamic Periodic Table – Cu, C, K


The MOLE!!?!?! -  A spicy sauce flavored with chocolate, usually served with turkey or chicken -  Any of various small insectivorous mammals, esp. of the family Talpidae, living chiefly underground, and having velvety fur, very small eyes, and strong forefeet. -  A spy who becomes part of and works from within the ranks of an enemy governmental staff or intelligence agency. -  A large, powerful machine for boring through earth or rock, used in the construction of tunnels -  A small, congenital spot or blemish on the human skin, usually of a dark color, slightly elevated -  A very large number: 6.022 x 1023 - The atomic or molecular weight of a substance expressed in grams


C. The Mole

Multiple Units •  One dozen eggs is twelve eggs •  One class is twenty students •  One kilometer is 1000 meters

•  One alphabet is …………? •  One Liter is ………..? •  One gross is………...?

Can you think of some multiple units?


C. The Mole

•  One mole of anything contains 6.022 x 1023 units of that substance. –  Avogadro’s number is 6.022 x 1023.

•  A sample of an element with a mass equal to that element’s average atomic mass (expressed in g) contains one mole of atoms.


C. The Mole


One marble has a diameter of 1cm and covers approx. one sq cm Area of USA is 10 million sq km = 107 sq km = 1017 sq cm Depth of marbles in a mole is 6 x 1023 / 1017 = 6 x 106 cm = 60 km

One Mole of marbles would cover the USA to a depth of approximately 60 km!!!


If you had a ball of string that was 1 mole of millimeters long would it reach around the Earth? (The circumference of the Earth is 40,076 km)


How many grams do the following weigh?

1.00 mol Se

2.00 mol Fe

72.5 mol Pb

0.102 mol Mg

0.521 mol Ni

1.23 x 10-3 mol Pt


26.2 g Gold

41.5 g Calcium

335 mg Barium

12.01 g Carbon

1.42 x 10-3 g Palladium

How many moles in the following samples?

(P211 Q16)


Relationships Between Moles, Mass and Atoms

Avogadro’s Number

6.022x1023

Atomic Mass from Periodic

Table

Moles Atoms Mass


Summary •  One amu is one-twelfth of the mass of a 12C atom •  One amu is close to the mass of one proton or

one neutron.

•  One amu is a very small mass – 1.66 x 10-24 g

•  One mole is 6.022 x 1023 units of anything •  One mole (of atoms) of an element will have a

mass in grams equal to the mass in amu of one atom of that element

•  “One Mole” refers to 6.022 x 1023 atoms or to the atomic mass expressed in grams of an element


Some Questions……… •  How many atoms are there in a bucket of water? How

could we determine that without having to count them all individually?

•  I know that one atom of carbon reacts with one molecule of oxygen to make one molecule of carbon dioxide. How many grams of oxygen react with 12 grams of carbon to make carbon dioxide? C + O2 reacts to give CO2

•  How are the atomic masses in the Periodic Table different to mass numbers?


1.  To understand the definition of molar mass 2.  To learn to convert between moles and mass 3.  To learn to calculate the mass percent of an element in

a compound

Objectives


A. Molar Mass

•  A compound is a collection of atoms bound together.

•  The molar mass of (“one mole of”) a compound is obtained by summing the masses of the appropriate number of moles of component atoms.

1 mole of methane 1 mole of

carbon and 4 moles of hydrogen


A. Molar Mass

•  For compounds containing ions the molar mass is obtained by summing the masses of the component ions. This is sometimes called the formula weight.


What is the Molar Mass of the following Compounds?

Na3N

CS2

NH4Br

C2H5OH

H2SO3

H2SO4

MgSO4

Ni3(PO4)2


A. Molar Mass

•  Moles of a compound (g) =

Calculations Using Molar Mass

mass of the sample (g)gmolar mass of the compound ( )

mol

•  Mass of a sample (g) = moles of sample (mol) x molar

mass of compound (g/mol)

•  Number of atoms in moles of a sample = number of moles x number of atoms in formula x Avogadro’s number


Relationships Between Moles, Mass and Atoms

Avogadro’s Number

6.022x1023

Atomic Mass from Periodic

Table

Moles Atoms Mass


Relationships Between Moles, Mass, Molecules and Atoms

Avogadro’s Number

6.022x1023

Molar Mass using Periodic

Table

Molecules Moles Mass

Atoms

Molecular Formula


B. Percent Composition of Compounds

•  Percent composition consists of the mass percent of each element in a compound:

mass of a given element in 1 mol of compound 100%mass of 1 mol of compound

× Mass percent =

What are the mass percentages of C, H and O in Ethanol?


Calculate the Mass Percent of These Compounds

LiBr

CuOH

SrSO4

C6H12O6

C2H4O

C4H8O2


1.  To understand the meaning of empirical formula 2.  To learn to calculate empirical formulas 3.  To learn to calculate the molecular formula of a

compound

Objectives


What is the Formula? Mass Spectrometry

•  If I analyze a compound by mass as 7.7% Hydrogen and 92.3 % Carbon what is its formula?

•  If I have 100g of the compound how many grams do I have of each element?

•  How many moles of each element do I have? •  What is the simple number ratio between the two

numbers of moles?

•  What is the formula?

•  Is this the only possible formula?


A. Empirical Formulas

•  The empirical formula of a compound is the simplest whole number ratio of the atoms present in the compound.

What is the empirical formula of the following compounds?

•  Sodium Peroxide Na2O2

•  Terephthalic acid C8H6O4

•  Phenobarbital C12H12N2O3

•  1,4-dichloro-2-butene C4H6Cl2


B. Calculation of Empirical Formulas


What is the Empirical Formula of a Compound Containing…..

0.0806g Carbon, 0.01353 g Hydrogen, 0.1074 g Oxygen?

P212 Q36-39


C. Calculation of Molecular Formulas

•  The molecular formula is the exact formula of the molecules present in a substance.

•  The molecular formula is always an integer multiple of the empirical formula. Molecular formula = (empirical formula)n

where n is a whole number

Empirical Formula: CH

Molar Mass = 78

Molecular Formula?


Formula Summary For the sugar glucose

Documents

Atoms & Moles; Molar Mass & % Composition; … 6 Atoms & Moles; Molar Mass & % Composition; Formulas of Compounds A. Counting by Weighing • We can count objects by knowing how much