Learning Molar Mass

Surekha Waldia

Critical thinking, Problem Solving
Remember, Understand, Application

Learning Molar Mass
Students introduced to Molar MassLearn How to Find the Molar Mass for CompoundsLearn to find the atomic mass from the periodic table
The underlying concept of in teaching to calculate molar mass is making students reconnect with Unitary method- concept learned in Mathematics in class 5th or 6th
Periodic Table
Pencil and paper Black board chalks

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)


What do you think will be the unit of measurement for weighing atoms?
How do we derive the unit grams/mole
Accurate, Clearly explained, Knowledgeable, Methodical, Precise, Organized

It is very important to explain students the concept that mole is another unit of measuring space that atoms acquire in a particular area. Rather than going ahead mechanically it is better if examples of measurements are used to explain students this new unit of measuring matter.

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