Relative Molecular Mass

Mass and Moles

10RT Review

Let's start with Relative Atomic Mass

This is a measure of the overall mass of a single atom. The mass of an electron is negligible and so when we measure Atomic mass only the Neutrons and Protons contribute

Multiple Choice

The Atomic Mass is expressed on the periodic table by the mass number. What is the atomic mass of Chlorine?

• 17

• 35

• 18

• .45

But wait a minute...why is there a decimal place? We can't have 0.45 of a proton or a neutron

This is where the 'relative' part of Relative Atomic Mass comes in

Multiple Choice

What's different about these two molecules of Chlorine?

• the number of electrons

• the number of protons

• the number of neutrons

In nature, we have isotopes!

Isotopes are atoms of an element that contain different amounts of neutrons within the nucleus. Remember, neutrons have weight so we have to calculate the average. To do this, we use something called percentage abundance

Poll

See what they did here?

• making sense

• I'm shaky but I see it

• I still don't get it

But what about the mass of a compound?

When dealing with more than one atom, we can total up the masses of each atom to find the 'Relative Molecular Mass' or the weight of the entire molecule

Multiple Choice

First things first...how do we read a molecular formula? This little number is also called a subscript but what does it mean?

• it balances the equation

• it's how many atoms are in the entire molecule

• it's how many of the atom in front of it are present

Poll

Subscripts tell us how many of the atom in front of it are present in the molecule. So when we calculate Relative Formula Mass we know how many to include in our calculations

• Got it!

• I still don't see it

Multiple Choice

Try it! Calculate the molecular mass of the pictured compound using the mass number information below:

C-12

S- 32

Na- 23

H-1

O-16

• 124

• 119

• 133

• 142

That's cool and all but...where do moles come in?

Now we can start tying Atomic Mass and Formula mass into moles

Multiple Select

Which of the following are true about moles?

• it is a unit of measurement

• it's a type of rat

• it's Avogadro's constant

• it means 6.02 x 10²³

• they live in the ground

Moles are the standard unit of measurement in chemistry. It tells us how many molecules/atoms/ions of a substance we have

• 1 mole is equal to 6.02x10²³ of whatever we're measuring (atoms/ ions/molecules)

• This number is considered Avogadro's constant

But what does that have to do with Relative Atomic Mass and relative Formula Mass?

Well if we can calculate Relative Atomic Mass and Relative Formula Mass...we can calculate one mole of the substance! Because one mole of a substance is equal to it's mass expressed in grams

Poll

According to the periodic table, the atomic mass of carbon is 12 therefore 12 grams of carbon is the same as 1 mole of carbon

• I see it

• Not feeling it

Multiple Choice

So how many grams are there in 1 mole of Argon?

• 39.9g

• 18g

• 21g

• 57g

Multiple Choice

And how many molecules are in 1 mole of Argon?

• 39.9

• 18

• 3.01 x 10¹²

• 6.02x10²³

Now that we can calculate moles AND mass, we can convert between them

We can derive formulas to convert between mass and moles if one is unknown.