The Mole – A measurement of matter 10-1 Notes The Mole – A measurement of matter

Measuring Matter Chemistry is a quantitative science We will be analyzing the composition of samples of matter and perform chemical calculations that relate quantities of reactants in a chemical reaction to the quantities of products Ex: How many grams of H2 and N2 must be combined to make 200 grams of fertilizer (NH3)? 1st we must find a way to measure the amount of matter we have. By counting how much matter, finding the mass, or finding the volume of matter

What is a Mole? Review: Matter is composed of atoms, molecules, and ions Particles that are much to small to count We use a unit called a mole (mol) to count particles of matter A mole of a substance is 6.02 x 1023 representative particles of that substance Known as Avogadro’s Constant see table 10.1 Ex. 1 mole of Na = 6.02 x 1023 atoms of Na

Mole Conversions How many moles of Mg are in 1.25 x 1023 atoms of Mg? Use this conversion: 1 mol = 6.02 x 1023 Answer: 0.208 mol Mg Practice Problem # 3 How many atoms are in 2.12 mol of propane? Conversion – moles  molecules  atoms Answer: 1.40 x 1025 atoms Practice Problem #5

The Mass of a Mole Review: The atomic mass of an element is expressed in atomic mass units (amu). The atomic masses are relative values based on the most common isotope of carbon. The atomic mass of C is 12 amu which is 12 times heavier then H (mass of 1 amu) Results in a constant 12:1 ratio Therefore, 12 grams of C and 1 gram of H must contain the same number of atoms

The Mass of a Mole The atomic mass of an element expressed in grams is the mass of a mole of the element Ex. The molar mass of Carbon is 12 grams Because 1 mole of any element contains 6.02 x 1023 atoms, the number of atoms in 12 grams of Carbon = the number of atoms in 16 grams of Oxygen Molar Mass is the mass of 1 mol of atoms of any element

The Mass of a Mole Step 1: Determine the formula for the compound Step 2: Find the number of grams of each element in one mole of the compound Step 3: Add the masses of the elements in the compound This method is used for any type of compound

The Mass of a Mole Find the mass of 1 mol of Sulfur trioxide SO3 = 1 sulfur atom and 3 oxygen atoms 1 atom sulfur x 32.1 amu = 32.1 amu 3 atoms oxygen x 16.0 amu (per oxygen) = 48.0 amu 32.1 amu + 48.0 amu = 80.1 amu for Sulfur trioxide Substitute grams for amu. 1 mol of SO3 has a mass of 80.1 grams Practice Problem # 7

Mole-Mass and Mole-Volume Relationships 10-2 Notes Mole-Mass and Mole-Volume Relationships

Mole-Mass Relationship Use the molar mass of an element or compound to convert between the mass of a substance and the moles of that substance. Mass (grams) = number of moles x (mass / 1 mole) Ex. The molar mass of NaCl is 58.5 grams. What is the molar mass of 3 moles of NaCl? 3.00 mol NaCl x 58.5g/1 mol = 176 g Sample problem 10.5

Mole-Mass Relationship Suppose in the lab you were able to obtain 10.0 grams of Na2SO4. How many moles is this? moles = mass x (1mol/mass) moles of Na2SO4 = 10.0 grams x (1 mol/142.1 g) moles of Na2SO4 = 7.04 x 10-2 Sample problem 10.6

Mole-Volume Relationship The volumes of one mole of different solid and liquid substances are not the same fig. 10.7 on page 295 The volumes of gases are much more predictable In 1811 Avogadro stated that equal volumes of gases at the same temperature and pressure contain equal number of particles Even though the particles that make up different gases are not the same size, they take up relatively the same space

Mole-Volume Relationship The volume of a gas will vary depending on the temperature or the pressure Increase in pressure = decrease in volume The volume of a gas is usually measured at standard temperature and pressure (STP) STP is at O oC and at a pressure of 101.3 kPa or 1 atmosphere (atm) At STP 1 mol of any gas occupies a volume of 22.4 L This is referred to as molar volume

Mole-Volume Relationship To calculate volume of a gas at STP: Volume of gas = moles of gas x (22.4 L/1 mol) Determine the volume (Liters) of 0.60 mol SO2 at STP You can also calculate molar mass from Density. The density of a gas at STP is 22.4L/mol Molar mass = density @ STP x molar volume @ STP Determine the mass of a compound that has a density of 1.964 g/L at STP

Percent Composition and Chemical Formulas 10-3 Notes Percent Composition and Chemical Formulas

Percent Composition The % composition of a compound consists of a % value for each different element in the compound % composition from mass data The % mass of an element is the # of grams of the element divided by the mass in grams of the compound, multiplied by 100 % mass of element = mass of element x 100 mass of compound Sample problem 10.9

Percent Composition % Composition from the Chemical Formula % mass = Mass of element in 1 mol compound x 100 Molar mass of compound Sample Problem 10.10

% Composition as a Conversion Factor You can use % comp. to calculate the number of grams of any element in a specific mass of a compound Multiply the mass of the compound by a conversion factor based on the % comp of the element in the compound. Sample problem: In 10.10 we found in propane, the % comp of Carbon was 81.8%. What would the mass of Carbon be in an 82.0 g sample?

Empirical Formulas The % comp. of any compound is the data we use to calculate the basic ratio of elements contained in a compound The basic ratio that gives the lowest whole-number ratio of the atoms of the elements in a compound is called the empirical formula An empirical formula may or may not be the same as a molecular formula Example: The molecular formula for hydrogen peroxide is H2O2 the empirical formula is HO Sample problem 10.11

Molecular Formulas The molecular formula of a compound is either the same as its experimentally determined empirical formula, or it is a simple whole-number multiple of its empirical formula To find the molecular formula you must first know the molar mass of the compound. Sample Problem 10.12