Chap 3Chap 3

The math of chemistryThe math of chemistry

Formula massFormula mass

Atoms have weight.Atoms have weight.

We measured these as atomic mass.We measured these as atomic mass.

Ex: Oxygen weighs 16 AMUEx: Oxygen weighs 16 AMU

Carbon weighs 12 AMUCarbon weighs 12 AMU

Formula massFormula massBut what about when we have But what about when we have different amounts of atoms in a different amounts of atoms in a compound?compound?

The rule isThe rule isThe weight of the molecule is equal The weight of the molecule is equal to the weight of each of the to the weight of each of the components in the molecule added components in the molecule added up.up.

Formula massFormula mass

Ex. COEx. CO22 – C= 12 AMUC= 12 AMU– O=16 AMU X 2= 32O=16 AMU X 2= 32– 32+12= 44 AMU32+12= 44 AMU

Gram massGram mass

The formula mass measures the The formula mass measures the weight of 1 tiny little atom.weight of 1 tiny little atom.

But how about if we want to measure But how about if we want to measure a gazillion atoms.a gazillion atoms.

The weight of a gazillion atoms is The weight of a gazillion atoms is called the called the gram formula massgram formula mass

Gram formula massGram formula mass

To find the gram formula massTo find the gram formula mass

we find the weight in AMU’s we find the weight in AMU’s then add the word gramsthen add the word grams

Gram formula mass of COGram formula mass of CO22 = 44 = 44 gramsgrams

Percent CompositionPercent Composition

If we have a mixture of any two If we have a mixture of any two things and we want to know what things and we want to know what percent is each part….percent is each part….

We find the mass of the entire thing, We find the mass of the entire thing, the mass of the part we want to the mass of the part we want to focus on… and dividefocus on… and divide

Percent compositionPercent composition

Ex. What percent is oxygen in HEx. What percent is oxygen in H2200

The whole thing weights 18 AMUThe whole thing weights 18 AMU

Oxygen weight 16 AMUOxygen weight 16 AMU

16/18 X 100 =89 %16/18 X 100 =89 %

The MoleThe Mole

In life we used words to describe In life we used words to describe large unitslarge units

A dozen = 12 ( so 2 dozen = 24)A dozen = 12 ( so 2 dozen = 24)

A century = 100 ( so 2 centuries = A century = 100 ( so 2 centuries = 200)200)

In chemistry we use the term MOLEIn chemistry we use the term MOLE

A MOLEA MOLE

A mole just means a really really big A mole just means a really really big amount of something. amount of something.

(602200000000000000000000.0 of (602200000000000000000000.0 of something to be exact).something to be exact).

A mole of a substance weighs as A mole of a substance weighs as much as the gram formula mass much as the gram formula mass ( this = the mass just in grams)( this = the mass just in grams)

MolesMoles

Find the weight of a mol ofFind the weight of a mol of

CC22HH66

OO33

OO22

MolesMoles

If a dozen apples weighs 10 pounds If a dozen apples weighs 10 pounds how much does 3 dozen apples how much does 3 dozen apples weigh?weigh?

To do this we multiply 3 X 10 = 30To do this we multiply 3 X 10 = 30

So use this to determine how much 2 So use this to determine how much 2 moles of Hmoles of H22O weightO weight

MolesMoles

Find the weight of:Find the weight of:

2 Moles of C2 Moles of C66HH1212OO66

4 Moles of NaCl4 Moles of NaCl

MolesMoles

So far we determined how many So far we determined how many grams we had from knowing how grams we had from knowing how many moles we had (that was many moles we had (that was easy)easy)

What if we want to know how many What if we want to know how many moles we have in an amount of moles we have in an amount of gram?gram?

MolesMoles

For instance:For instance:

How many moles are in 4.75 grams How many moles are in 4.75 grams of Sodium hydroxide ( NaOH)of Sodium hydroxide ( NaOH)

To do this look at the back page of To do this look at the back page of your reference table and follow the your reference table and follow the formula givenformula given

MolesMoles

Find the gram-formula mass of NaOHFind the gram-formula mass of NaOH

Na= 23Na= 23

O=16O=16

H= 1H= 1

23+16+1=40 grams23+16+1=40 grams

4.75/40 =0.119 mol4.75/40 =0.119 mol

When we have an equation for a When we have an equation for a reaction of different moleulcules, the reaction of different moleulcules, the molecules have coefficents molecules have coefficents (numbers in front of them) (numbers in front of them)

This tells us how many of each we This tells us how many of each we need.need.

exampleexample

For a tricycle 3 wheels + 2 For a tricycle 3 wheels + 2 handlebars + 1 frame = a tricyclehandlebars + 1 frame = a tricycle

So we could write So we could write

3W +2Hb +1 F 3W +2Hb +1 F 1 Tricycle 1 Tricycle

In equations it’s the same thingIn equations it’s the same thing

Here we see 2 Magnesium's react with Here we see 2 Magnesium's react with 1 O1 O2 2 to create 2 magnesium Oxidesto create 2 magnesium Oxides

EquationsEquations

Once we know the basic equations Once we know the basic equations we can see if one thing changes so we can see if one thing changes so do the other.do the other.

Ex: 8 eggs + 3 sugars +1 flour Ex: 8 eggs + 3 sugars +1 flour 10 cookies 10 cookies

If I double what comes in I will double If I double what comes in I will double what comes outwhat comes out

EquationsEquations

2 C2 C22HH66 + 7O + 7O22---- 4CO 4CO22 + 6 H + 6 H22OO

2 7 4 62 7 4 6

4 14 8 124 14 8 12

1 3.5 2 31 3.5 2 3