unit v the mole concept. v.1 avogadro’s hypothesis avogadro’s hypothesis : equal volumes of...

UNIT VThe Mole Concept

V.1 AVOGADRO’S HYPOTHESIS Avogadro’s hypothesis: Equal volumes of

different gases contain the same number of particles (at the same temperature and pressure).

If 1L of gas A reacts with 1L of gas B, then the formula for the compound is AB

If 2 L of gas A reacts with 1L of gas B, then the formula for the compound is A2B

If 2 L of gas A reacts with 3L of gas B, then the formula for the compound is A2B3

Questions: p. 78 #2-5

V.2 THE MOLE

1. The Mole the number of carbon atoms in exactly 12 g

of carbon. = 6.02 x 1023 carbon atoms in 12 g of C

(Avogadro’s number) Ex: 1 mol of Mg atoms = ______________ atoms

and weighs _________ g the number 6.02 x 1023 is an accepted

standard for the MOLE

V.2 THE MOLE

2. Molar Mass the mass of ONE MOLE of particles. molar mass is also known as atomic mass the periodic table shows the atomic mass of

elementsthese are measured in amu (atomic mass

units)the masses shown on the periodic table can

be used directly if they are measured in GRAMS

in other words, the atomic mass of an element is the same as how many grams of it there would be in one mole (we call this molar mass).

V.2 THE MOLE

Ex: Element Atomic Mass Molar Mass Ca N

Since the molar mass is the MASS (g) of ONE MOLE (mol) of particles:

the units of molar mass are g/mol

Ex: 1 mol of Carbon = 12 grams, so molar mass of carbon = 12.0 g/mol 

V.2 THE MOLE

More examples:  Chlorine (Cl) = 35.5 g/mol

 Oxygen (O2) =

 Sodium (Na) =

 Sodium oxide (Na2O) =

 Tin (II) cyanide (Sn(CN)2) = Questions: p. 80 #6 ,7 (odds)

V.2 THE MOLE

3. Mass Conversions  Molar Mass as a Conversion Factor  Molar mass allows us to calculate or convert: mass → # of moles # of moles → mass

Molar mass is given in grams per mole (g/mol); for example Carbon has a molar mass of 12.0 g/mol

Therefore, 1 mol OR 12.0 g are conversion factors 12.0 g 1 mol

V.2 THE MOLE

Ex: How many moles of Carbon are there in 22 grams of Carbon?

Ex: How many grams of Carbon are there in 3.00 mol of Carbon?

V.2 THE MOLE

Ex: What is the mass of 4.85 mol of Nitrogen?

Ex: What is the mass of 2.90 mol of H2?

V.2 THE MOLE

Ex: How many moles of H2SO4 are there in 4.0 grams of H2SO4?

Ex: How many grams are there in 3.00 x 10-4

moles of CaCl2?

Questions: p. 82 # 8 -10 (odd)

V.2 THE MOLE

4. Moles and the Volume of Gas  Recall Avogadro’s hypothesis:

Equal volumes of different gases contain the same number of particles (at the same temperature and pressure).

Standard Temperature and Pressure  Chemists have a set STANDARD for

temperature and pressure: STP = standard temperature and pressure STP = 0°C and 101.3 kPa

V.2 THE MOLE

5. Molar Volume  The molar volume of a gas is the VOLUME

occupied by ONE MOLE of the gas. It is a fact that at STP, 1 mol of any gas

occupies 22.4 L of volumethis is known as ‘molar volume’

We then get another CONVERSION FACTOR1 mol OR 22.4L

22.4 L 1 mol

BUT ONLY AT STP

V.2 THE MOLE

Ex: How many L of space will 2.3 mol of He (g) occupy at STP?

Ex: How many mol of Ne (g) will fill a 400L space at STP?

Questions p 83 #11, 12

V.2 THE MOLE The mole is the fundamental unit in chemistry for

measuring the AMOUNT OF SUBSTANCE. We can convert from moles to number of particles.

1 mole = 6.02 x 1023 

CONVERSION FACTOR: 1 mol OR 6.02 x 1023

6.02 x 1023 1 mol

** there are no units for Avogadro’s number because it is simply a number. It’s like a dozen (a dozen = 12).

Think of it as the chemist’s dozen (but a much bigger number!)

V.2 THE MOLE

Ex: How many moles of NaBr are there in 1.7 x 1025 molecules of NaBr?

Ex: How many molecules of LiCl are there in 0.5 moles of LiCl?

Questions: p. 84 # 15- 18 (odd), 19 and 20

V.3 MULTIPLE CONVERSIONS

First: we must know how to find the # of atoms in a given # of molecules 

Ex: How many atoms are there in 15 molecules of CuSO4 · 5H2O?

Question: p. 85 # 21(a,c)

V.3 MULTIPLE CONVERSIONS

CONVERSION CONVERSION FACTOR

Moles ↔ # of Particles (Avogadro’s number)

Moles ↔ Mass (Molar mass)

Moles ↔ Volume (gas @ STP)

(Molar Volume)

Molecules ↔ Atoms

The MOLE is CENTRAL to all conversions between mass, volume and particles. 

THE MOLE FLOWER

V.3 MULTIPLE CONVERSIONS

Ex: Find the mass, in grams, of 1.25L of NH3 (g) at STP:

Ex: What is the volume occupied by 45.0g of KOH at STP?

V.3 MULTIPLE CONVERSIONS

Ex: What is the mass, in grams, of 175 N (Nitrogen) atoms?

V.3 MULTIPLE CONVERSIONS

What to do if density is mentioned in the question:

d = m/V

 If the volume of a liquid or solid is unknown....use V = m/d (you can always find the mass from the moles of substance present)

 Note: you cannot use 22.4L when calculating a volume of liquid or solid (only gases!)

V.3 MULTIPLE CONVERSIONS

Ex: What is the volume occupied by 5.00 mol of ethanol, CH3 CH2 OH(l)?

(the density of ethanol is 0.790 g/mL)

V.3 MULTIPLE CONVERSIONS

If the number of moles is unknown...use the density and volume to calculate mass m = d x V and then convert mass to moles

V.3 MULTIPLE CONVERSIONS

Ex: How many moles of Hg (l) are contained in 56 mL of Hg(l)? (d = 13.6 g/mL)

V.3 MULTIPLE CONVERSIONS

Ex: CCl4 (l) has a density of 1.59 g/mL. How many atoms of C are in 200 mL of CCl4?

V.3 MULTIPLE CONVERSIONS

Ex: What is the density of CH2F2 (g) at STP?

V.3 MULTIPLE CONVERSIONS

If the molar mass of a gas at STP is unknown....find the density of the gas and then combine the density with volume of 1 mol (22.4L) to find the mass of 1 mole

V.3 MULTIPLE CONVERSIONS

Ex: A 3.0 L bulb contains 2.2 g of a gas at STP. What is the molar mass of the gas?

Questions: p. 88 # 25-34