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Part 4: Molar Relationships The mole and molar calculations Stoichiometry Solution Concentrations

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- Part 4: Molar Relationships
- The mole and molar calculations
- Stoichiometry
- Solution Concentrations
- Acid/Base Theory

You will need a calculator and periodic table to complete this section.

The Mole and Mole Calculations

One mole = 6.02 x 1023 representative particles

One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure

One mole = the atomic mass listed on the periodic table.

For example: one mole of Helium contains 6.02 x 1023 atoms of Helium and it has a mass of 4.00260 grams. At 0°C and one atmosphere of pressure, it would occupy 22.4 Liters.

- Sample problem: How many liters would 2.0 moles of Neon occupy?
- Answer:
- 2.0 moles Ne x 22.4 Liters Ne = 44.8 Liters Ne
- 1.0 moles Ne

- Sample problem: How many moles are in 15.2 grams of Lithium?
- Answer:
- 15.2 g Li x 1 mole Li = 2.19 mole Li
- 6.941 g Li

The Mole and Mole Calculations

One mole = 6.02 x 1023 representative particles

One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure

One mole = the atomic mass listed on the periodic table.

- Sample problem: How many liters would 14 grams of Helium occupy?
- Answer:
- 14 g He x 1 mole He x 22.4 L He = 78 Liters He
- 4.0026 g He 1 mole He

The Mole and Mole Calculations

One mole = 6.02 x 1023 representative particles

One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure

One mole = the atomic mass listed on the periodic table.

You try one:

What is the mass of 9.0 Liters of Argon gas at 0°C and one atmosphere of pressure?

The Mole and Mole Calculations

One mole = 6.02 x 1023 representative particles

One mole = 22.4 Liters of gas at 0°C and one atmosphere of pressure

One mole = the atomic mass listed on the periodic table.

You try one:

What is the mass of 9.0 Liters of Argon gas at 0°C and one atmosphere of pressure?

9.0 L Ar x 1 mol Ar x 39.948 g Ar = 16 g Ar

22.4 L Ar 1 mole Ar

The Mole and Mole Calculations

The molar mass = the sum of all the atomic masses.

Example Ca(NO3)2 = 40.08 + 2(14.01) + 6(16.00) = 164.10 grams

You try one:

What is the gram formula mass (molar mass) of Mg3(PO4)2?

The Mole and Mole Calculations

The molar mass = the sum of all the atomic masses.

Example Ca(NO3)2 = 40.08 + 2(14.01) + 6(16.00) = 164.10 grams

You try one:

What is the gram formula mass (molar mass) of Mg3(PO4)2?

3(24.305) + 2(30.97376) + 8(15.9994) = 262.86 grams

The Mole and Mole Calculations

The molar mass = the sum of all the atomic masses.

Example Ca(NO3)2 = 40.08 + 2(14.01) + 6(16.00) = 164.10 grams

You try one:

What is the gram formula mass (molar mass) of Mg3(PO4)2?

3(24.305) + 2(30.97376) + 8(15.9994) = 262.86 grams

What is the percent Magnesium in Mg3(PO4)2?

Answer: 3(24.305) x 100 = 27.7%

262.86

You try one:

What is the percent Lithium in Li2SiO3?

molar mass = 2(6.941) + 28.0855 + 3(15.9994) = 89.9657 g

% Li = 2(6.941) x 100 = 15.4%

89.9657

A Brief Return to Empirical Formulas

Empirical Formulas are the reduced form of Molecular formulas.

For example: The empirical formula for C5H10 is CH2.

A favorite exam question:

What is the empirical formula of a compound that contains 30% Nitrogen and 70% Oxygen?

This is really a percent composition problem. Figure out which compound contains 30% nitrogen.

a) N2O

b) NO2

c) N2O5

d) NO

The Mole and Mole Calculations

- At Standard Temperature and Pressure (STP) 1 mole of gas = 22.4 L
- You can use this to calculate the density of a gas in g/Liter at STP.
- Example: What is the density of CO2 gas at STP?
- The molar mass of CO2 = 12.0111 + 2(15.9994) = 44.0099 g
- Density = mass/volume = 44.0099 g/22.4 L = 1.96 g/L

The Mole and Mole Calculations

- At Standard Temperature and Pressure (STP) 1 mole of gas = 22.4 L
- You can use this to calculate the density of a gas in g/Liter at STP.
- Example: What is the density of CO2 gas at STP?
- The molar mass of CO2 = 12.0111 + 2(15.9994) = 44.0099 g
- Density = mass/volume = 44.0099 g/22.4 L = 1.96 g/L
- You try one:
- What is the density of Cl2 gas at STP?

The Mole and Mole Calculations

- At Standard Temperature and Pressure (STP) 1 mole of gas = 22.4 L
- You can use this to calculate the density of a gas in g/Liter at STP.
- Example: What is the density of CO2 gas at STP?
- The molar mass of CO2 = 12.0111 + 2(15.9994) = 44.0099 g
- Density = mass/volume = 44.0099 g/22.4 L = 1.96 g/L
- You try one:
- What is the density of Cl2 gas at STP?
- Answer: molar mass = 2(35.453) = 70.906 g
- 70.906 g/22.4 L = 3.165 g/L

Stoichiometry

For reaction calculations, the molar ratio is used.

Example:

How many moles of nitrogen will react with 9 moles of hydrogen to produce ammonia according to this equation?

2N2(g) +3 H2(g) → 3NH3(g)

Given: 9 moles H2, Find moles N2

For reaction calculations, the molar ratio is used.

Example 2:

How many grams of nitrogen are needed to react with 2.0 grams of hydrogen using this equation?

2N2(g) +3 H2(g) → 3NH3(g)

Given: 2.0 grams H2, Find grams N2

- Calculating molarity:
- Memorize this equation: Molarity = moles/liters or M = mol/L
- Memorize conversion factor: 1000 mL = 1 L
- Some example of using this equation:
- Example 1: the molarity of 2.0 moles of HCl in a 0.50 L solution of water is:
- molarity = 2.0 mole HCl/0.50 L = 4.0 Molar or 4 M
- Example 2: The molarity of 0.40 moles of HCl in a 300. mL L solution of water is:
- molarity = 0.40 moles HCl/0.300. L = = 1.3 M

- Example 3:
- The molarity of 72.9 g of HCl in 5.0 liters of aqueous solution is:
- Answer: first calculate the moles of HCl

Then calculate molarity of solution:

2.00 mol HCl/5.0 L = 0.40 M HCl

- You try one:
- What is the molarity of 1.2 grams LiF in a 50. mL aqeous solution?
- Answer: first calculate the moles of LiF

Then calculate molarity of solution (remember convert mL to Liters):

0.046 mol LiF/0.050 L = 0.95 M LiF

- Diluting concentrated solutions
- Memorize: M1V1 = M2V2
- M1 and V1 are the beginning molarities and volumes
- M2 and V2 are the ending molarities and volumes
- V1 and V2 can be in Liters or mLs, but must be the same units for both
- Example:
- What is the molarity of a 10. mL sample of 2.0 M aqueous HCl diluted to 40. mL
- Answer:
- (2.0)(10.) = (M2)(40.) so M2 = 0.5 Molar HCl

- Diluting concentrated solutions
- Memorize: M1V1 = M2V2
- M1 and V1 are the beginning molarities and volumes
- M2 and V2 are the ending molarities and volumes
- V1 and V2 can be in Liters or mLs, but must be the same units for both
- You try one:
- How many milliliters of 6.0 Molar HCl are required to prepare 240 mL of 2.0 Molar HCl?
- Answer:
- (6.0)(V1) = (2.0)(240) so V1 = 80. mL HCl

Exothermic and Endothermic Reactions

Catalysts lower the Activation energy barrier, making reactions faster.

Acids and Bases

Generic formula for acids = HX (HCl, HNO3, H2SO4)

Generic formula for bases = MOH where M is any metal (NaOH, KOH, Ca(OH)2 Ammonia, NH3, is also a base.

Acid solutions have a pH less than 7

Basic solutions have a pH more than 7

Arrhenius acids:

Taste _______

turn litmus paper red.

Arrhenius bases

Taste _______

Feel __________

Turn litmus paper blue.

SAFETY NOTES

Always add acid to water when diluting

If you spill acid or base on yourself, rinse with lots of water.

sour

bitter

slippery

- What is pH?
- pH indicates the hydrogen ion molarity [H+] in a solution
- pH = -log[H+]
- pOH indicates the hydroxide ion molarity [OH-] in a solution.
- pOH = -log[OH-]
- Example: A 1.0 x 10-3 molar solution of HCl would have a pH of ___
- Example: A 1.0 x 10-4 molar solution of KOH would have a pOH of ___
- Memorize: pH + pOH = 14.
- Example: A solution with a pH of 8 will have a pOH of: ____.

3

4

6

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