Mole CalculationsLesson 3

Avogadro’s number

Review 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  particles or molecules or atoms!

CONVERSION FACTOR

1 mol OR 6.02 x 1023

6.02 x 1023 1 mol

•This is Avogadro's number

•There are no set units for Avogadro’s number because it is simply a number. You can use any of the units(particles, molecules or atoms) depending on the question.

Molar mass from Periodic Table

Molar Mass Units

# grams or 1 mole

1 mole # grams

Particles Moles Grams

1.00 mole = 6.02 x 1023 particles Molar Mass

From Periodic Table g / mole

The molar mass is a new conversion factor that can convert between grams and moles.

Avogadro’s number is another conversion factor that can convert between moles and particles

TryEx: 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?

Try Ex: how many grams are in 3 moles of Al2S3

STPRecall 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 ºC and 101.3 kPa

Molar VolumeJust like we have a molar mass, we also have a 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 volume

– this is known as ‘molar volume’

• We then get another CONVERSION FACTOR

1 mol OR 22.4L

22.4 L 1 mol

BUT ONLY AT STP, and only for gases!

The Molar Volume

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

 

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

Find # of atoms in a molecule How many atoms are there in 1 molecule of CuSO4 · 5H2O?

 How many atoms are there in 15 molecules of CuSO4 · 5H2O? Always find the # of atoms in 1 molecule first, then multiply by # of molecules (15)

How many H atoms are in CuSO4 · 5H2O?

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.

 

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(g) at STP?

 

MULTIPLE CONVERSIONS

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

MULTIPLE CONVERSIONSWhat if you want to find the volume of a solid or a

liquid????Use densityd = 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)

 This is because….you cannot use 22.4L when calculating a volume of liquid or solid (only gases!)

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

(the density of ethanol is 0.790 g/mL)

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

MULTIPLE CONVERSIONSEx: How many moles of Hg (l) are contained in 56 mL of Hg(l)? (d =

13.6 g/mL)

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

200 mL of CCl4?

MULTIPLE CONVERSIONS

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

MULTIPLE CONVERSIONS

If the molar mass of a gas at STP is unknown ( in other words, you don’t have the formula for the gas)

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

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

Helpful hints - Write out what is given in the question with its units(on left)

- Write out what units your answer has to end up with(on right)

- Using the different conversion factors you know, start multiplying to cancel out the units you don’t want, and keep the ones you want.

Homework Page 88, 89, 90

#35, #36(b,d), #37 (d), #38(b), #39(d,f)

#40(a,f) #41(a,d,f, j) #42(a,c,e,i,l,s)

#43(a,d,g,i,k,o,g,r)

You must practice these questions, or you will be lost for this unit ….