Step 1: Explaining Molar Mass:
Substances take up space and have mass. Therefore, molecules also have mass because substances are made up of molecules and atoms.
Question that comes in mind: How do we normally measure molecules?What would be the value if measurement for them?
A mole is a unit of measure that helps us compare particles of any given substance and its mass.
Next question is how many atoms does a mole have?
1 mole = 6.022 x 10^23 atoms. Or this many atoms are packed in space which is equivalent to 1 mole.
The quantity is Avogadro’s number, named after the Italian mathematician and chemist.
Brainstorming Question : What do you think will be the unit of measurement for weighing atoms?
Since mass is measured in grams therefore Molar mass of a given element is also expressed in grams, or we can phrase it as weight of one mole or 6.022 x 10^23 atoms of that element is grams/mole
Example: Molar mass oxygen = 15.99 grams/mole
(Brainstorming question:How do we derive the unit grams/mole)
This means that 6.022*10^23 atoms of oxygen weigh=15.99 gram
Molar mass of Oxygen= 15.99grams/6.022*10^23 atoms
or to make it simpler
Molar Mass of Oxygen= 15.99grams/1 mole
Therefore Molar Mass Unit = grams/mole
Once we have explained to students the concept of Molar Mass Unit, we can move on to working out finding
1.The atomic masses of individual elements in the periodic table
The first thing we need is to find the individual atomic masses for each element given in the periodic table.The element sodium has an atomic/molar mass of 22.98976 g/mol. The element chlorine has an atomic/molar mass of 35.453 g/mol.
2: Count how many atoms there are for each element
For the compound sodium chloride, since there are no subscripts (small numbers at the bottom of each element’s symbol), that means there is only one sodium and only one chlorine atom for this compound.
3: Find the molar mass
Now that we know how many atoms there are for each element, we can find the molar mass.
Calculating Molar Mass of a Compound
The molecular mass of any molecule can be obtained by adding the molar masses of the elements in a compound, in the correct ratios. For example, the molecular mass of water (H2O) is the sum of the masses of two moles of hydrogen, plus one mole of oxygen.
molar mass hydrogen = 1.01 gram/mole
molar mass oxygen = 15.99 grams/mole
molecular mass water = 2.02 + 15.99 = 18.01 grams/mole
Converting Grams to Moles
Suppose you want to know how many moles of sodium are in 33 grams of sodium. The molar mass of sodium (chemical symbol Na) is 22.99 grams. Set up the conversion as follows:
? moles sodium = (1 mole sodium/22.99 g sodium) x 33 g sodium = 1.44 moles sodium
Formula: Mass (g) = mole x Molar Mass (g/mol)
Converting Moles to Grams
For example, suppose you want to know how many grams of silver are in .054 moles of silver. The molar mass of silver (chemical symbol Ag) is 107.87 grams/mole. Set up the conversion as follows:
? grams silver = (107.87 g silver / 1 mole silver) x .054 moles silver = 5.82 grams silver
Formula: Moles = Mass (g) .
Molar Mass (g/mol)