grouping! when with many items in a large set need to be counted, it is often useful to work with...
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Grouping!
• When with many items in a large set need to be counted, it is often useful to work with groups of items rather than individual items.
Item Quantity Amount
Gloves Pair 2
Cans of pop
Six-pack 6
Eggs Dozen 12
Pens Gross (12 dozen)
144
Paper Ream 500
• Chemists use a quantity to group atoms and molecules that is much larger than a dozen or a ream - THE MOLE (mol)
• The mole is defined as the amount of substance that contains as many entities (atoms, molecules, or units) as exactly 12 g of Carbon.
• One mole of a substance contains • 6.022 141 99 x 1023 particles
• This number is Avogadro’s constant and for the purposes of this class we will just know it as
• 6.022 x 1023 particles.
• The mass on the periodic table is called the molar mass.
• 55.847 g of iron is one mole.
• One mole of iron has 6.022 x 1023 atoms.
• So 55.847 g of iron has 6.022 x 1023 atoms.
Mole Conversions
• The Mole formula! Be able to rearrange it to solve for each of the variables!
n = m MM
Number of moles (mol)
mass (g)
Molar mass (g/mol)
Mole to Mass Example
• 2.001 mol KCl is how many grams?• First you need the molar mass of KCl from the
periodic table.• 39.0983 + 35.4527 = 74.5510 g/mol
• The you need to do the conversion.
• 2.001 mol x 74.5510 g = 149.2 g1 mol
Another Mole to Mass Example
• How many grams is in 0.42 mol of Mg(OH)2?
• Molar mass of Mg(OH)2 is• 24.3050 + 2(15.9994) + 2(1.00794) = 58.3197 g/mol
• 0.42 mol x58.3197 g = 24 g1 mol
Mass to Mole Example
• You are given 157 g NaOH. How many moles is this?
• To solve this, you need the molar mass from the periodic table.
• 22.989768 + 15.9994 + 1.00794 = 39.9971 g/mol
• 157 g NaOH x 1 mol = 3.93 mol 39.9971g
Another Mass to Mole example
• How many moles are in 1526 g K2CO3?
• Molar mass of K2CO3 is • 2(39.0983) + 12.011 + 3(15.9994) = 138.197 g/mol
• 1526 g x 1 mol = 11.04 mol138.197 g
Moles to Particles Example
• How many particles are in 10.8 mol FeO?• The only thing you need for this conversion is
Avogadro’s constant (6.022 x 1023)
• 10.8 mol FeO x 6.022 x 1023 particles1 mol
• 6.50 x 1024 particles
Another Mole to Particle Example
• How many atoms are in 17.6 mol of CaCl2?
• Avogadro’s constant is 6.022 x 1023
• 17.6 mol CaCl2 x 6.022 x 1023 atoms
1 mol• 1.06 x 1025 atoms
Moles to Volume at STP Example
• Volume at standard temperature (25oC) and pressure (1 atm) is a constant of 22.4 L.
• How many L are in 25.5 mol Al2O3?
• All you need to do the conversion is the constant of 22.4 L.
• 25.5 mol x 22.4 L = 571 L1 mol
Volume to Mole Example at STP
• How many moles in 6.85 L of Cl2 gas?
• All you need is the constant.
• 6.85 L x 1 mol = 0.306 mol22.4 L
• When you calculate and use the molar mass of a compound, you are making an important assumption.
• Say you are working with water, you are assuming that every sample of water contains two hydrogen and one oxygen and the mass ratio is 2(1.00794) to 1(15.9994).
• At this point, now that we have the periodic table, this two-to-one ratio seems obvious. But when scientists first discovered that compounds contained elements in fixed mass proportions, they did not have the periodic table.
• In fact, the discovery of fixed mass proportions was an important step toward the development of atomic theory.
FYI• There are an infinite number of
significant digits in a mole, so don’t assume it to have only 1!
• You MUST convert ALL quantities into standard units (L, g, etc) to use in all formulas
• Molar masses for molecules are calculated based on their FORMULA
• For full marks you MUST SHOW ME ALL STEPS, FORMULAS, INCLUDE FINAL STATEMENTS and correct SIGNIFICANT DIGITS
• My advise… carry your units through all calculations so that you can be sure your answer is correct when they cancel out properly