Stoichiometry The mathematics of Chemistry

What is Stoichiometry? The proportional relationship between two or more substances during a chemical reaction. The proportional relationship between two or more substances during a chemical reaction. In other words, using dimensional analysis to convert one substance to another In other words, using dimensional analysis to convert one substance to another There are many different types, but they are all similar. There are many different types, but they are all similar. So, let’s start small. How small? So, let’s start small. How small?

We know an atom is extremely small, but 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 Number of atoms in a penny: about or 2.97 x 10 22

How do we get that number? By using Avogadro's Number 6.02 x and using dimensional analysis again

What is Avogadro’s Number? It is 6.02 x atoms per mole It is 6.02 x atoms per mole What is a mole? What is a mole? The number of atoms in exactly 12 grams of carbon-12 and it is the SI unit for the amount of a substance The number of atoms in exactly 12 grams of carbon-12 and it is the SI unit for the amount of a substance Example problems Example problems –6.33 x atoms of C are how many moles? –How many atoms of Zn are in 3.5 moles?

The mole is also related to molar mass Molar Mass – the mass in grams of 1 mol of an element, ion, compound, etc. Represented by g/mol (what we did last class) Molar Mass – the mass in grams of 1 mol of an element, ion, compound, etc. Represented by g/mol (what we did last class) Example problems Example problems –92.6 g of CO 2 equals how many moles? –1.25 moles of S 8 equals how many grams?

Atomic Mass Units verses Molar Mass The mass in grams of 1 mol of an element is numerically equal to the element’s atomic mass from the periodic table in atomic mass units. The mass in grams of 1 mol of an element is numerically equal to the element’s atomic mass from the periodic table in atomic mass units. In other words, 1 g/mol = 1 amu In other words, 1 g/mol = 1 amu They are interchangeable They are interchangeable

Putting it all together. Let’s look at our penny again How did they get that number for the number of Cu atoms in a penny? How did they get that number for the number of Cu atoms in a penny? A penny has an average mass of 3.13 g. A penny has an average mass of 3.13 g. Copper weighs amu (g/mol), which we found on the periodic table Copper weighs amu (g/mol), which we found on the periodic table Here’s the game plan. We want to go from grams to atoms, but we have to always go through the mole. So, we will go from g  moles  atoms Here’s the game plan. We want to go from grams to atoms, but we have to always go through the mole. So, we will go from g  moles  atoms

3.13 g Cu x 1 mol Cu g Cu = mol Cu mol Cu x 6.02 x mol Cu = 2.97 x Cu atoms Example Problems

So, this is doing two types of Stoichiometry Problems. Atoms  moles Grams  moles

Summary Slide #1 How is a mole related to atoms? What is the magic number? What is the magic number? Avogadro’s number (N A )– 6.02 x atoms (molecules, ions, whatever)/mole Avogadro’s number (N A )– 6.02 x atoms (molecules, ions, whatever)/mole When going from moles to atoms (multiply): When going from moles to atoms (multiply): When going from atoms to moles (divide): When going from atoms to moles (divide): 6.02 x atoms 1 mol 6.02 x atoms

Summary Slide #2 How is the mole related to mass? Molar Mass Molar Mass For an element, it is the mass from the periodic table For an element, it is the mass from the periodic table For compounds, you must calculate the molecular weight of the compound. For compounds, you must calculate the molecular weight of the compound. Sometimes, before one can work with a mass, you have to use the density of the substance. (D=m/V or m=DV) Sometimes, before one can work with a mass, you have to use the density of the substance. (D=m/V or m=DV)

Summary Slide #3 How is a mole related to mass? When going from moles to grams (multiply): When going from moles to grams (multiply): When going from grams to moles (divide): When going from grams to moles (divide): molar mass 1 mol molar mass

Example Problems 1) How many moles are in g of PO 4 -3 ? 2) What is the mass of 6 moles of CH 2 O? 3) How many ions are in 0.25 moles of Cu +2 ? 4) How many moles are 8.25 x molecules of Na 2 CO 3 ? 5) How many formula units are in 6.34 g of NaCl? 6) How many grams are in 3.4 x molecules of CH 4 ? 7) How many moles are in 10 mL of water (density of water = 1 g/mL)?

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