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Announcements Online HW #2 (Type 2) due

tomorrow by 7:00 p.m.

Lab write-up for lab this week is not due until next Wednesday, September 19th.

Avogadro’s number Just as:

12 “things” = dozen

2 “things” = pair

3 “things” = trio

For any atom:

1.008 g hydrogen atoms = 6.022x1023 hydrogen atoms = 1 mole hydrogen atoms

12.01 g carbon atoms = 6.022x1023 carbon atoms = 1 mole carbon atoms

Avogadro’s number Avogadro’s number can be used for not only

atoms, but molecules as well

18.016g H2O = 6.022x1023 molecules H2O =

1 mole H2O

Remember that Avogadro’s number is just 6.022x1023 “things”!

Molar Mass The average relative masses on the

periodic table can be related to grams per mole (g/mol) 1 mole of O = 16.00g, 1 mole of N = 14.01g

This is what we refer to as molar mass

Clicker #1 You have two beakers on your lab table. Beaker #1 contains 32.07 g of sulfur and Beaker #2 contains 74.92 g of arsenic (As). Which beaker contains the greatest number of atoms? Choose the best answer.

A) Beaker #1 because one sulfur atom weighs less than one arsenic atom so you need more of the sulfur atoms to fill the beaker. B) Beaker #1 because it contains more moles of sulfur atoms versus the number of moles of arsenic atoms in Beaker #2. C) Beaker #2 because arsenic has a greater mass than sulfur. D) Beaker #2 because it contains more moles of arsenic atoms versus the number of moles of sulfur atoms in Beaker #1. E) Beakers #1 and #2 contain the same number of atoms because there is one mole in each.

Conversions Using Avogadro’s number and molar

mass, we can convert from atoms (or molecules) to grams:

Atoms (or molecules) moles grams

Example How many H2O molecules are

contained in 50.0 g of water? How many hydrogen atoms are present in this sample?

Example: Methane

Molecular formula: CH4

Clicker #2

Does a methane molecule (CH4) consist of more hydrogen or carbon?

A) hydrogen

B) carbon

C) depends

Example: Methane Why does it depend?

4 out of 5 of the atoms are hydrogen (80% by molar ratios)

But chemists are more interested in % by mass

% mass = [(mass of part)/(mass of whole)] *

100

Example: Methane Take apart methane molecule:

CH4: 1C + 4H = 1*(12.01g) + 4*(1.008g) = 16.042 g/mol

C: 12.01g / 16.042g * 100 = 74.9% C

H: 4*(1.008) / 16.042g * 100 = 25.1% H

74.9% + 25.1% = 100%

Clicker #3

What is the percent composition (by mass) of carbon in 2 moles of CH4?

A) 2.00% C

B) 37.45% C

C) 74.9% C

D) 149.8% C

Percent Composition C: 2(12.01g) / 2(16.042g) * 100 = 74.9% C

H: 2[4(1.008)] / 2(16.042g) * 100 = 25.1% H

Same thing as 1 mole of CH4!

*which brings us to the point: percent composition is independent of the amount*

Law of definite proportions Notice when percent composition of

a substance does not change, you have a compound. A compound consists of the same percent composition by mass

This is what we define as the Law of Definite Proportions

Percent Composition Why is percent

composition important?

Example: You might analyze a sample to see the percent composition of the elements to see if it matches a drug like cocaine

Percent Composition What is the % by mass of oxygen in

each?

H2O vs. H2O2

16.00/18.016 * 100 = 88.9%

32.00/34.016 *100 = 94.1%

Genie in a Bottle DEMO: Calling upon a genie!

What’s happening here?

H2O2 H2O + O2

Used a catalyst to speed

up the reaction

Percent Composition Notice that % by mass of O is not

that different, yet H2O and H2O2 have very different properties

Similar percent compositions does not mean the properties will be the same!

Example You have some “nitrogen oxide”

compound and you want to figure out what it is (both the formula and the name). You know it’s 30.4% nitrogen by mass.

NxOy = formula?

name?

Example Remember, the amount doesn’t matter

with percent composition (1 versus 2 methane molecules)

What are percentages out of?

100

So it’s easiest to assume that you have 100 grams of your compound (100 g of NxOy)

Example 30.4g N in every 100g of compound

30.4g/100g * 100 = 30.4% N

Could assume that you have 60.8g N in 200g compound too

60.8g/200g * 100 = 30.4% N (still get the same %)

Example So again, it’s easiest to start with 100g of

compound

Amount of oxygen: 100-30.4 = 69.6g

Mass ratio: 30.4g N / 69.6g O

Is the formula then N30.4O69.6?

No! A chemical formula represents the number of atoms in a compound (not the mass of each)

Example We must convert the masses to moles of

atoms

30.4g N * 1 mol N/14.01g N = 2.17 mol N

69.6g O * 1 mol O/16.00g O = 4.35 mol O

To find whole number ratio, divide both by smaller number:

2.17/2.17 = 1 N 4.35/2.17 = 2 O

**So the formula is NO2, and the compound is nitrogen dioxide**

Example But wait, is the molecular formula really

just nitrogen dioxide?

Find the % composition of dinitrogen tetroxide

N2O4 = 2N + 4O = 2(14.01) + 4(16.00) = 92.02 g/mol

(28.02/92.02) * 100 = 30.4% N

(64.00/92.02) * 100 = 69.6% O

Ratio is the same as NO2!

Example If we took a sample of N2O4 and

experimentally determined its nitrogen content, then calculated the formula as we just did, we would think that the compound is NO2 instead of N2O4

We need a way to differentiate between the two, and to do that we would need the molar mass of the compound

Molecular Formula Molecular formula: actual formula of the

compound (N2O4, N3O6, N4O8)

Empirical formula: lowest ratio of compound formula (NO2)

Clicker #4 The molecular formula (actual formula) for water is H2O. What is its empirical formula?

A) HO½

B) H2O

C) H2O2

D) H4O2

E) Cannot be determined without knowing the percent mass of each.

Molecular Formula For some compounds, the empirical

formula and molecular formula are the same thing (H2O)

Ionic compounds (metals and nonmetals bonded together) always have the same empirical and molecular formula because they are not really bonded together, like nonmetal-nonmetal compounds are

Percent by Mass DEMO: sugar and H2SO4

Record your observations

DEMO: sugar and KClO3

Record your observations

Percent by Mass If you have equal masses of each

compound, which one has the greatest number of oxygen atoms?

H2SO4 C12H22O11 (sugar) KClO3 (potassium

chlorate)

98.086 g/mol 342.296 g/mol 122.551 g/mol

Example Fe + O2 ”iron oxide”

A reaction of a sample of 12.00g Fe 17.16g “iron oxide” compound

What is the correct formula and name of this iron oxide?

Example The molar mass of propane is 44.094

g/mol

(hydrocarbon – made up of H and C)

You know that you have 36.03 g C. What is the molecular formula of propane?

Example You have a compound composed of C, H,

O:

40%C, 6.71% H

and molar mass of 120 g/mol

What is the molecular formula? (CxHyOz)

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