Chap 3 The math of chemistry

Formula mass Atoms have weight. We measured these as atomic mass. Ex: Oxygen weighs 16 AMU Ex: Oxygen weighs 16 AMU Carbon weighs 12 AMU Carbon weighs 12 AMU

Formula mass But what about when we have different amounts of atoms in a compound? The rule is The rule is The weight of the molecule is equal to the weight of each of the components in the molecule added up.

Formula mass Ex. CO 2 –C= 12 AMU –O=16 AMU X 2= 32 –32+12= 44 AMU

Gram mass The formula mass measures the weight of 1 tiny little atom. But how about if we want to measure a gazillion atoms. The weight of a gazillion atoms is called the gram formula mass

Gram formula mass To find the gram formula mass we find the weight in AMU’s then add the word grams Gram formula mass of CO 2 = 44 grams

Percent Composition If we have a mixture of any two things and we want to know what percent is each part…. We find the mass of the entire thing, the mass of the part we want to focus on… and divide

Percent composition Ex. What percent is oxygen in H 2 0 The whole thing weights 18 AMU Oxygen weight 16 AMU 16/18 X 100 =89 %

The Mole In life we used words to describe large units A dozen = 12 ( so 2 dozen = 24) A century = 100 ( so 2 centuries = 200) In chemistry we use the term MOLE

A MOLE A mole just means a really really big amount of something. ( of something to be exact). A mole of a substance weighs as much as the gram formula mass ( this = the mass just in grams)

Moles Find the weight of a mol of C 2 H 6 O 3 O 2

Moles If a dozen apples weighs 10 pounds how much does 3 dozen apples weigh? To do this we multiply 3 X 10 = 30 So use this to determine how much 2 moles of H 2 O weight

Moles Find the weight of: 2 Moles of C 6 H 12 O 6 4 Moles of NaCl

Moles So far we determined how many grams we had from knowing how many moles we had (that was easy) What if we want to know how many moles we have in an amount of gram?

Moles For instance: How many moles are in 4.75 grams of Sodium hydroxide ( NaOH) To do this look at the back page of your reference table and follow the formula given

Moles Find the gram-formula mass of NaOH Na= 23 O=16 H= =40 grams 4.75/40 =0.119 mol

When we have an equation for a reaction of different moleulcules, the molecules have coefficents (numbers in front of them) This tells us how many of each we need.

example For a tricycle 3 wheels + 2 handlebars + 1 frame = a tricycle So we could write 3W +2Hb +1 F  1 Tricycle

In equations it’s the same thing

Here we see 2 Magnesium's react with 1 O 2 to create 2 magnesium Oxides

Equations Once we know the basic equations we can see if one thing changes so do the other. Ex: 8 eggs + 3 sugars +1 flour 10 cookies If I double what comes in I will double what comes out

Equations 2 C 2 H 6 + 7O 2 --  4CO H 2 O