StoichiometryStoichiometry

The mathematics of The mathematics of ChemistryChemistry

What is What is StoichiometryStoichiometry??

The The proportionalproportional relationship between relationship between two or more substances during a two or more substances during a chemicalchemical reaction. reaction.

In other words, using dimensional In other words, using dimensional analysis to analysis to convertconvert one substance to one substance to anotheranother

There are many different types, but There are many different types, but they are all similar.they are all similar.

So, let’s start small. How small?So, let’s start small. How small?

We know an atom is We know an atom is extremely small, but extremely small, but

how small is an atom?how small is an atom?In other words, how small is the

smallest particle of an element that retains the properties of the element?

World Population: about 6000000000Number of atoms in a penny:

about 29700000000000000000000 or 2.97 x 1022

How do we get that How do we get that number?number?

By using By using Avogadro'sAvogadro's Number Number

6.02 x 106.02 x 102323

and using and using dimensionaldimensional analysisanalysis again again

What is What is Avogadro’sAvogadro’s Number?Number?

It is It is 6.02 x 106.02 x 102323 atoms per moleatoms per mole What is a What is a molemole?? The number of atoms in exactly The number of atoms in exactly 1212

grams of grams of carbon-12carbon-12 and it is the SI and it is the SI unit for the amount of a unit for the amount of a substancesubstance

Example problemsExample problems– 6.33 x 106.33 x 102424 atoms of C are how many atoms of C are how many

moles?moles?– How many atoms of Zn are in 3.5 moles?How many atoms of Zn are in 3.5 moles?

The The molemole is also related to is also related to molar massmolar mass

MolarMolar MassMass – the mass in grams of 1 – the mass in grams of 1 mol of an element, ion, compound, mol of an element, ion, compound, etc. Represented by etc. Represented by g/molg/mol (what we (what we did last class)did last class)

Example problemsExample problems– 92.6 g of CO92.6 g of CO22 equals how many moles? equals how many moles?

– 1.25 moles of S1.25 moles of S88 equals how many equals how many grams? grams?

Atomic Mass Units verses Atomic Mass Units verses Molar MassMolar Mass

The mass in grams of 1 mol of an The mass in grams of 1 mol of an element is element is numericallynumerically equalequal to the to the element’s atomic mass from the element’s atomic mass from the periodic table in atomic mass units.periodic table in atomic mass units.

In other words, In other words, 11 g/mol = g/mol = 11 amu amu They are They are interchangeableinterchangeable

Putting it all together. Let’s Putting it all together. Let’s look at our penny againlook at our penny again

How did they get that number for the How did they get that number for the number of Cu atoms in a penny?number of Cu atoms in a penny?

A penny has an average mass of A penny has an average mass of 3.13 3.13 gg..

Copper weighs Copper weighs 63.54663.546 amu (g/mol), amu (g/mol), which we found on the which we found on the periodic tableperiodic table

Here’s the game plan. We want to go Here’s the game plan. We want to go from from gramsgrams to to atomsatoms, but we have to , but we have to always go through the always go through the molemole. So, we . So, we will go from will go from gg molesmoles atomsatoms

3.13 g Cu x1 mol Cu

63.546 g Cu= 0.049255 mol Cu

0.049255 mol Cu x6.02 x 1023

1 mol Cu= 2.97 x 1022 Cu atoms

Example ProblemsExample Problems

So, this is doing two So, this is doing two types of Stoichiometry types of Stoichiometry

Problems.Problems.

Atoms Atoms moles moles

Grams Grams moles moles

Summary Slide #1Summary Slide #1How is a mole related to How is a mole related to

atoms?atoms? What is the magic number?What is the magic number? Avogadro’s number (NAvogadro’s number (NAA)– 6.02 x 10)– 6.02 x 102323

atoms (molecules, ions, whatever)/moleatoms (molecules, ions, whatever)/mole When going from moles to atoms When going from moles to atoms

(multiply):(multiply):

When going from atoms to moles When going from atoms to moles (divide):(divide):

6.02 x 106.02 x 102323 atoms atoms

1 mol1 mol

1 mol1 mol6.02 x 106.02 x 102323 atoms atoms

Summary Slide #2Summary Slide #2How is the mole related to How is the mole related to

mass?mass? Molar MassMolar Mass For an For an elementelement, it is the mass from the , it is the mass from the

periodic tableperiodic table For For compoundscompounds, you must calculate , you must calculate

the molecular weight of the compound.the molecular weight of the compound. Sometimes, before one can work with Sometimes, before one can work with

a mass, you have to use the a mass, you have to use the densitydensity of of the substance. (the substance. (D=m/VD=m/V or or m=DVm=DV))

Summary Slide #3Summary Slide #3How is a mole related to mass?How is a mole related to mass? When going from moles to grams When going from moles to grams

(multiply):(multiply):

When going from grams to moles When going from grams to moles (divide):(divide):

molar massmolar mass

1 mol1 mol

1 mol1 molmolar massmolar mass

Example ProblemsExample Problems1)1) How many moles are in 10.23 g of POHow many moles are in 10.23 g of PO44

-3-3??

2)2) What is the mass of 6 moles of CHWhat is the mass of 6 moles of CH22O? O?

3)3) How many ions are in 0.25 moles of CuHow many ions are in 0.25 moles of Cu+2+2? ?

4)4) How many moles are 8.25 x 10How many moles are 8.25 x 102828 molecules molecules of Naof Na22COCO33? ?

5)5) How many formula units are in 6.34 g of How many formula units are in 6.34 g of NaCl?NaCl?

6)6) How many grams are in 3.4 x 10How many grams are in 3.4 x 102424 molecules of CHmolecules of CH44??

7)7) How many moles are in 10 mL of water How many moles are in 10 mL of water (density of water = 1 g/mL)?(density of water = 1 g/mL)?

HomeworkHomework

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