College Preparatory Chemistry Unit 6 –Chemical Composition

Chemical Composition We’ve been doing conversions and we will continue to do so in this unit. However, now we will be converting grams and atoms and molecules and volume (of a gas) using a chemistry conversion called the mole, an SI unit for counting.

The Concept of Counting by Weighing You walk into a candy store and get 500 grams of jellybeans. You want to get the same number of mints as you do jellybeans. You could count them all, but there is an easier way. The store clerk knows the average mass of a jellybean is 5 g and the average mass of a mint is 15 g.   500 g jellybeans x 1 jellybean = 100 jellybeans 1 5 g

The Concept of Counting by Weighing Each mint weighs on average 3x more than each jellybean; therefore, you will need 1500 g of mints to have the same number of each candy.   100 mints x 15 g mint = 1500 g mints 1 1 mint

The Concept of Counting by Weighing This idea of having two samples of different substances but each having the same number of particles is important in chemistry. The method described above is called counting by weighing.

Counting by weighing weigh the mass of the entire sample, find the average mass of one unit, and then divide the mass of one unit into the entire mass of the sample   In everyday life, there are “words” that mean “numbers”: 12 = dozen 13 = baker’s dozen 144 = gross 500 = ream

The Mole In chemistry, scientists use a word called the mole to represent 6.022 x 1023. This is an extremely large number !!!!! 1 mole of marbles will cover the entire earth to a depth of __50__ miles If there were 6.022 x 1023 people on the earth, each person would have _1/100,000_ square inch(es) to stand on (assume you can stand on every part of the earth) 6.022 x 1023 pieces of paper stacked together would reach __Beyond our solar system or to the moon and back 80 billion times_

HOW BIG IS THE MOLE How Big Is The Mole

The Mole Conversion Factor: 1 mole of a substance = 6.022 x 1023 particles of that substance The particles are the smallest unit you can have of that substance. Atoms if you are talking about an element (such as atoms of carbon)molecules if you are talking about a molecule (such as molecules of water)

The Mole The symbol for mole is mol. The word “mole” comes from the German word for molecular weight. We will use the term “molar mass” instead of “molecular weight”. 6.022 x 1023 is called Avogadro’s number, named after Amedeo Avogadro, an Italian physicist. This number is measured AND not an exact amount. We usually round this to three significant figures and use 6.02 x 1023

1 Mol of different substances

The Mole Mole Day: Celebrated each year on Oct 23rd from 6:02 am to 6:02 pm   Scientists must use counting by weighing in order to count atoms (or other particles). How can we find the average mass of an atom? Use the average atomic mass which is found on the periodic table Conversion Factor: 1 mole of an element = its average atomic mass in grams The average atomic mass is a measured number AND not an exact amount.

Molar Mass When using the periodic table, you can round all average atomic masses to two decimal places - but you must round correctly! Ex. On the periodic table, 1 mole of oxygen weighs 15.999 g; rounded to two decimal places it is 16.00 g (you need these precision zeros) Ex. On the periodic table, 1 mole of potassium weighs 39.098 g; rounded to two decimal places it is __39.10 g____ Ex. On the periodic table, 1 mole of carbon weighs 12.011 g; rounded to two decimal places it is __12.01 g__

Mole Problems

Mole Problems Ex. How many moles are in 0.500 g of lead?   0.500 g Pb x ­1 mol Pb = 0.00241 mol Pb 1 207.2 g Pb NOTE: Make sure you “double-label” your answer (unit and substance) since the average atomic mass of one element is NOT the same as the average atomic mass of another element. You will have points deducted if you do not double-label. You will thank me for this later! (Well, maybe you will.)

Examples Ex. How many atoms are in 2.5 mol of tungsten? 2.5 mol W x 6.02 x 1023 atoms = 1.5 x 1024 atoms W 1 1 mol W

Examples Ex. How many moles are in 5.75 x1024 molecules of methane? 5.75 x1024 molecules of CH4 x ______1 mole CH4 = 9.55 moles CH4 1 6.02 x 1023 molecules CH4

Hint: Helpful Hint: We have a problem here with the units for moles and molecules. The abbreviation for mole is “mol” and molecule does not have an official abbreviation. You must use something other than “mol”. Suggestions are ____mlc____ or ____molecule____.

TWO-STEP CONVERSIONS Last Unit we saw that if we were doing a metric to metric conversion, we used the base unit (such as grams or liters) as the link between the two units in a two-step conversion.   In this chapter, the mole is used as the link between particles and mass in a two-step conversion.

Example Ex. How many atoms are in 10.0 g of aluminum? 10.0 g Al x 1 mol Al x 6.02 x 1023 atoms = 2.23 x 1023 atoms Al 1 26.98 g Al 1 mol Al HINT: If mole is not the given OR the unknown, you need a two-step conversion.

Molar Volume Unlike solids or liquids, the volume of a gas is affected by pressure and temperature. Because of this, scientists have determined a standard for temperature and pressure of gases.   STP- stands for Standard Temperature and Pressure; Temperature: __0ᵒC__ or ___273K___ Pressure: __1 atm___ or ___760.00mmHg___

Molar Volume Avogadro proposed that at the same temperature and pressure, equal volumes of gases will have the same number of particles. Molar Volume – the volume of 1 mole of any gas (at STP conditions) is 22.4 L Conversion Factor: 1 mole of any gas (at STP) = 22.4 L

Notes NOTE: This is a measured amount with 3SF not a definition.  

Gases Examples of three types of gases: Monatomic- Ar, He, Ne, etc   Monatomic- Ar, He, Ne, etc Diatomic- H2, O2, N2, Cl2, etc Polyatomic- CO2, NO2, CH4, etc

Molar Volume Problems Ex. A container with a volume of 694 L contains how many moles of oxygen at STP? 694 L O2 x 1 mol O2 = 30.98214286 mol O2 = 31.0 mol O2 1 22.4 L O2 Watch rounding! You must add a precision zero in this example!

Ex. A chemical reaction produces 138 mol of HBr gas Ex. A chemical reaction produces 138 mol of HBr gas. What volume will that gas occupy at STP? 138 mol HBr x 22.4 L HBr = 3091.2 L HBr = 3090 L HBr 1 1 mol HBr Watch rounding! You must add a placeholder in this example!

Example Ex. What is the volume of 25.0 mol of helium gas at STP?   25.0 mol He x 22.4L He = 56.0 L He   1 1 mol He

Example 2 Ex. What is the volume of 275 grams of neon at STP? HINT: Remember, If mole is not the given OR the unknown, you need a two-step conversion. 275 g Ne x 1 mol Ne x 22.4 L Ne = 305.25273 = 305 L Ne 1 20.18 g Ne 1 mol Ne

Example 3 Ex. How many molecules of carbon monoxide are present in 100.L of the gas at STP? 100.L CO2 x 1 mol CO x 6.02x1023 molecule CO = 1 22.4L CO 1 mol CO 2.69x1024 molecules CO

Molar Mass Molar Mass - the mass of 1 mole of a substance (in grams) Ex. Sodium = 22.99 g/mol (the average atomic mass - from the periodic table) Ex. Chlorine = 35.45 g/mol (the average atomic mass - from the periodic table)

Example Ex. Sodium chloride = ??? (sodium chloride is not on the periodic table!) But sodium and chlorine are!   Write the chemical formula to determine the type and amount of each element. Look up average atomic masses, multiply by amount present and add together. Sodium chloride = NaCl 1 Na = 22.99 g 1 Cl = 35.45 g Molar Mass = 58.44 g/mol Remember to watch decimal places when adding or subtracting! NOTE: The label g/mol means “grams per mole” or 58.44 grams per 1 mole (58.44g/ 1 mol). You will use this as a conversion factor.

Example Ex. Determine the molar mass of hydrogen gas. Remember, this is a diatomic molecule! H2 1.01 g + 1.01 g = 2.02 g/mol HINT: Keep Two Decimal Places When Calculating Molar Mass

Example 3 Ex. Determine the molar mass of sulfur dioxide. Sulfur dioxide = SO2 1 S = 32.07 g 2 O = 16.00 g x 2 = 32.00 g 64.07 g/mol

Example 2 Ex. Determine the molar mass of calcium hydroxide. Calcium hydroxide = Ca(OH)2   1 Ca = 40.08 g 2 O = 16.00 g x 2 = 32.00 g 2 H = 1.01 g x 2 = 2.02 g 74.10 g/mol

Example 4 Ex. Determine the molar mass of ammonium phosphide. ammonium phosphide = (NH4)3P 3 N = 14.01 g x 3 = 42.03 g 12 H = 1.01 g x 12 = 12.12 g P = 30.97 g 85.12 g/mol

How we use Molar Mass in calculations: Determine the molar mass of the substance (from the correct formula) Solve the problem using dimensional analysis   Ex. How many moles are in 26.0 g of magnesium chloride? a. MgCl2 = 24.31 g + 2(35.45 g) = 95.21 g/mol b. 26.0 g MgCl2 x 1 mol MgCl2 = 0.273 mol MgCl2 1 95.21g MgCl2

Example Ex. How many molecules are in 15.3 g of silicon hexafluoride? a. Si = 28.09 g F = 6 (19.00) = 114.00 g 142.09g/mol b. 15.3g SiF6 x 1mol SiF6 x 6.02x1023 molecules SiF6 = 1 142.09g SiF6 1 mol SiF6 = 6.48x1022 molecules SiF6

Example 2 Ex. What is the mass of 5.0 L of oxygen gas at STP (in grams)?   O2 2 O = 2 x 16.00 g = 32.00 g 5.0 L O2 x 1 mol O2 x 32.00 g O2 = 7.1 g O 1 22.4 L O2 1 mol O2

Now that we know how to do mole to molecule conversions, we can approach it a little differently. We are going to consider not only a mole of molecules, but also the atoms within the molecule.

Before we do that, let’s consider a non-chemistry example Non-chemistry - Example A: If you have a traditional married couple, you have two adults. They come as a couple, but they are two individual people, a man and a woman. 1 couple = 1 man and 1 woman 2 couples = _2_ men and _ 2 _ women 12 couples = _ 12 _ men and _ 12 _ women 1 dozen couples = ___ 1 dozen ___ men and ___ 1 dozen ___ women

Chemistry - Example B: IF you have a binary ionic molecule, you actually have two ions. They come as a molecule, but you have two individual ions. Remember you don’t see the charges any more because they are “zeroed out.” Let’s say the binary ionic molecule is NaCl. It is composed of two different ions: a Na+ ion and a Cl- ion. Follow the workup below.    1 molecule NaCl = 1 Na+ ion and 1 Cl- ion 2 molecule NaCl = _2__ Na+ ions and _2__ Cl- ions 12 molecule NaCl = _12__ Na+ ions and _12__ Cl- ions 1 dozen molecules NaCl = __ 1 dozen ____ Na+ ions and ___ 1 dozen ___ Cl- ions 6.02 x 1023 molecules NaCl = ____ 6.02 x 1023 __ Na+ ions and ____ 6.02 x 1023 ___ Cl- ions 1 mole NaCl = ____ 1 mole _____ Na+ ions and _____ 1 mole ______ Cl- ions

Non-chemistry - Example C: Analogy:

Chemistry - Example D: IF you have a water molecule, you actually have three atoms. They come as a molecule, but you have one atom of oxygen and two atoms of hydrogen. Follow the workup below. 1 molecule H2O = 1 O atom and 2 H atoms 2 molecule H2O = ___ O atoms and ___ H atoms 12 molecule H2O = ___ O atoms and ___ H atoms 1 dozen molecules H2O = _______________ O atoms and _________________ H atoms 6.02 x 1023 molecules H2O = _______________ O atoms and _________________ H atoms 1 mole H2O = _______________ O atoms and _________________ H atoms

Using the molecule and atom/ion relationship: Ex. How many atoms of hydrogen are in 15.0 grams of H2O?   First must calculate the molar mass of H2O: (2 x 1.01 g) + (1 x 16.00 g) = 18.02 g/mol 15.0 g H2O x 1 mole H2O x 6.02 x 1023 molecules H2O x _2 atoms H___ = 1.00 x 1024 1 18.02 g H2O 1 mole H2O 1 molecule H2O atoms H

Example Ex. How many moles of H+ are in 7.25 moles of hydrochloric acid?   Formula: ___HCl____ 7.25 moles HCl x __1 mole H+___ = 7.25 moles H+ 1 1 mole HCl

Example again Ex. How many ions of H+ (a different question) are in 7.25 moles of hydrochloric acid?    7.25 moles HCl x _1 mole H+_ x 6.02 x 1023 ions H+ = 4.36 x 1024 ions H+ 1 1 mole HCl 1 mole H+

Example 2 Ex. How many Cl- ions are in 18.5 g of magnesium chloride? First we must write the formula and then calculate the molar mass of magnesium chloride. Remember, the compound is ionic so it is made of ions, even though you do not see charges (since they are zeroed out).    Formula: __MgCl2_____    (1 x 24.31 g) + (2 x 35.45 g) = 95.21 g/mol   18.5 g MgCl2 x 1 mole MgCl2 x 2 mole Cl- x 6.02 x 1023 ions Cl- = 2.34 x 1023 1 95.21 g MgCl2 1 mole MgCl2 1 mole Cl- ions Cl-