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A Brief Demonstration about Moles - A chemistry Concept and A Part of Syllabus for Cambridge International Examination O Levels Checkpoint.
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Chemistry - MolesA Demonstration to describe the concept of moles more briefly.
Part 1: Chemical Quantities
Introduction
● How did you determine the number of M&Ms in the bag without actually counting?
Atoms
● Can you determine the number of atoms of an element even though you can not actually count them?
● Yes… IF there is a conversion factor that allows you to convert between mass and number of atoms.
Moles
● The SI base unit that measures amount of substance
● Avogadro’s Number:6.02 x 1023 atoms
Moles and Atomic Mass
● The atomic mass of an element in grams contains one mole of atoms (Avogadro’s number of atoms)
● Example: There are 6.02 x 1023 (or 1.0 moles) of atoms in 12.011 g of carbon
Moles as Conversion Factors
● Use moles/grams (or grams/moles) as a conversion factor
● Ex: 16.0 g oxygen -----------------------------
1 mole oxygen
Example 1
● How many moles of helium atoms are in 6.8 grams?
Example 2
● How many grams is equal to 1.51 moles of iron?
Examples 3, 4, and 5
● 17.6 grams of chlorine is equal to how many moles?
● How many moles of nickel do you have if you have 65.1 g?
● React 1.9 moles of sodium with chlorine gas to get sodium chloride. How many grams of sodium do you need?
Calculating Molar Mass
● Atomic Mass of an element = “molar mass”
● Molar mass of a compound must be calculated
Calculating Molar Mass
● How many oxygens are in a DOZEN water molecules?
● How many hydrogens?
Calculating Molar Mass
● One mole of WATER MOLECULES (H2O) is one mole of oxygen atoms and two moles of hydrogen atoms
● Calculate the mass of one mole of water molecules
Calculate the molar mass of…
● Boron Trifluoride
● Sodium Iodide
● Sodium Bicarbonate (NaHCO3)
● Glucose (C6H12O6)
● Copper (III) Sulfate
REMEMBER!
● Molar mass is the mass of one mole of an element/compound.
● Molar mass has units of grams/mole.
● Molar mass can be used as a conversion factor to find grams or find moles.
What you should know so far:●Explain the concept of a “mole” and give a definition
●Explain how one mole of an element is related to atomic mass
●Find the molar mass of an element (using correct units)
●Determine the number of grams/moles of a given element using molar mass
●Calculate the molar mass of a compound
●Determine the number of grams/moles of a given compound using molar mass
Making A Solution
● You want to make a solution by dissolving 4.00 moles of potassium nitrate in water. How many grams of the compound will you need?
Example!
● Calculate the number of moles in a 13.8-g sample of butane lighter fluid. The formula for butane is C4H10.
Find the mass of:
●0.702 mol Be.
●2.40 mol N2.
●0.160 mol H2O2.
●5.08 mol NaNO3.
Find the numberof moles.
●187 g Al
●0.264 gLi2HPO4
●333 g SnF2
● How many grams are in 5.66 mol of CaCO3?
● How many moles are in 508 grams of ethanol, CH3CH2OH?
Part 2: Moles of a Gas
Gases
● Solids/liquids are packed tightly together and more space is needed if atoms/molecules are larger
● Atoms/molecules in gases are far apart, so larger atoms/molecules do not need more space
Mole-Volume Relationship
● Avogadro’s Hypothesis – equal volumes of gases contain equal numbers of particles
● 1 mole of any gas takes up the same volume as long as…
STP
● …as long as the gases are at the same pressures and temperatures
● STP, or “standard temperature and pressure” means 0oC and 1 atmosphere, which is air pressure at sea level
Calculating Moles of a Gas
● At STP, a mole of any gas takes up 22.4 liters.
● Use 22.4 L/mol as a conversion factor!
Example 1
● How many liters will a 0.19-mole sample of oxygen gas take up?
Example 2
● A blimp has a volume of 95 kL (95,000 L). If it is filled with helium at STP, how many moles of the gas are contained within the blimp?
Examples 3 and 4
● Find the number of moles in a sample of chlorine gas stored in a 14.2-liter tank at STP.
● What is the volume of 0.18 moles of xenon at STP?
During Last Week’s Episode…
● Look up molar mass of elements / calculate molar mass of compounds
● Convert moles-grams using molar mass as the conversion factor (elements and
compounds)
● Convert moles-liters of a gas using molar volume (22.4 L/mol)
MOLES
Mass (g)
Volume (L) # Atoms
Conversion Factor: molar mass
Conversion Factor: molar volume(22.4 L / 1 mol)
Conversion Factor: Avogadro’s number
6.02 x 1023 atoms/mol
Challenge Problem
● Calculate the mass of 16.4 L of chlorine gas at STP.
Part 3: Molarity
Molarity
Molarity is a measure of how concentrated a solution is
Amount of solute per volume of solution
High concentration / high molarity
MolarityMeasured in units of moles/liter
mol/L = “M”
A 2.5 M solution has 2.5 moles of salt in one liter of water
Molarity Problems
To make one liter of a 2.5 molar solution of sodium chloride, how many grams of sodium chloride do you need?
Molarity Problems
SET MOLARITY PROBLEMS UP AS A PROPORTION!
●How many moles of silver nitrate (AgNO3) are in 0.4 L of a 6.0 M solution?
●How many grams of silver nitrate is that?
Molarity Problem #3
How many grams of lead(IV) chloride must be dissolved in 50 mL of water to make a 0.1 M solution?
Remember: 0.1 M = 0.1 mol/L
Molarity Problem #4
●If you dissolve 56 g of sugar (sucrose, C12H22O11) into a liter of water, what is the molarity of the solution you created?
Part 3: Percent Composition
Section 10.3, page 305
● Methane, CH4, is made of solely hydrogen and carbon. What percent of the compound is hydrogen? What percent is carbon?
Percent Composition
● Remember that some atoms are larger, by mass, than others
● Mass of each atom must be taken into account when calculating percent composition
Percent Composition
● Percent composition = % (by mass) of each atom in a compound
mass of element% composition = --------------------------- x 100%
mass of compound
Example 1
● 15.1 grams of oxygen combine with 1.9 g of hydrogen to make water. Find the percent composition of each element.
Example 2
● A sample of glucose is synthesized from 58.00 g carbon, 9.73 g hydrogen, and 77.27 g oxygen. Find the percent composition for each element.
Percent Composition and Chemical Formulas
● Allows you to find the %composition of an elementin a compound withoutknowing the mass of thatelement
molar mass of element% composition = --------------------------------------- x 100%
molar mass of compound
Example 1
● What percentage of a sample of table salt, sodium chloride, is sodium?
Example 2
● Determine the percent composition of iron (III) oxide.
Example 3
● Hypochlorous acid is one of the active sanitizers in pool-cleaning products. Its formula is HClO. What percent of the compound is oxygen?
Using Percent Compositionto Calculate Mass
● To calculate mass of one element in a specific mass of a compound, multiply the mass of the compound by the percent composition of that element.
Using Percent Compositionto Calculate Mass
● Example:
● Carbon is 27.3% of CO2. Of 45.1 g of CO2, what mass is carbon?
27.3% x 45.1 g = 12.3 g
Example 1
● Find the number of grams of sodium in 10.1 g of sodium sulfate.
● HINT! First write the formula for sodium sulfate, then find the % sodium.
Thank You
A Presentation by Irfad Imtiaz
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