Chemical Quantities Chapter 10

Measuring Matter Matter can be measured in three ways By mass By volume By counting

Measuring by counting The mass of a penny is 2.500 g You collect pennies in an empty 2-L bottle with a mass of 77.00 g. After a few months, the mass of your collection is 2.297 kg. How many pennies do you have? The mass of a nickel is 5.000 g If you have 625 g of nickels, how many pennies do you need to equal the mass of nickels?

Measuring by counting The mass of a penny is 2.500 g The mass of a nickel is 5.000 g What is the ratio of the masses of the coins? If you have 625 g of nickels, how many pennies do you need to equal the mass of nickels?

The Mole A dozen is a counting unit How many eggs are in 12 dozen? 42 eggs is how many dozen eggs? The mole is the SI unit for quantity of a substance It is a counting unit One mole contains 6.02 x 1023 represetative particles 1 mol O2 = 6.02 x 1023 O2 molecules 1 mol Na+ ions = 6.02 x 1023 Na+ ions How many atoms are in 1.5 mol He?

Mass and Moles The atomic mass of an element in amu’s = the mass of one mole of the element in grams. The atomic mass of Ne is 20.18 amu The molar mass of Ne is 20.18 g What is the molar mass of Ca? What is the molar mass of N2?

Molar Mass of a Compound Molar mass is the mass of one mole of a substance. Molar mass of a compound is the total sum of the molar masses of the elements in the compound The molar mass of H2O is… Element amu x # atoms H 1.01 x 2 = 2.02 O 16.00 x 1 16.00 Sum 18.02 g/mol

Mole-Mass Relationships Molar mass is the mass of one mole of a substance Molar mass is used to convert between moles and mass Find the mass of 3.00 mol of NaCl Find the number of moles in 86.68 g of NaCl

Mole-Volume Relationships Avogadro’s hypothesis: volume of a gas is proportional to the moles of a gas V α n Standard temperture and pressure T = 00C = 273.15K; P = 1 atm At STP, one mole of any gas occupies 22.4 L How many moles O2 are in 33.6 L of the gas at STP? Find the volume of 3.5 mol of N2 at STP.

Molar Mass & Density of Gases Remember that density is the ratio of mass to volume Gases are not very dense, so the units are grams/liter The molar mass of a gas can be determined using the standard molar volume of gas and the density of the gas

Molar Mass & Density of Gases A gas containing carbon and oxygen has a density of 1.964 g/L at STP. Find its molar mass. Find the density of krypton gas at STP. Find the density of a gas with a molar mass of 80 g/mol.

Mole Road Map

Percent Composition The law of definite proportions states a substance will always have the same proportions of elements by mass This means that each substance has a known percent composition by mass % comp is the mass of an element within a compound divided by the molar mass of the compound Example: %N in NH3 = mass of N/molar mass of NH3 %N = 14.01 g/17.04 g =0.822 = 82.2% %H = 1-0.882 = 0.178 = 17.8%

Percent Compositon Practice Determine % composition of glucose, C6H12O6 Find the % composition of potassium dichromate, K2Cr2O7

Empirical Formulas Empirical formulas show the smallest whole number ratio of elements in compound. Examples: CH CH4 CH2O HO SO3 In all these examples, the ratios of elements cannot be reduced

Determining Empirical Formula Empirical formulas can be determined from percent composition data Percent composition can be used as a conversion factor Example: a compound is 25.9% N and 74.1% O. Determine its empirical formula

Determining Empirical Formula from Percent Composition Convert % composition to moles N1.85 O4.63 ????? Element % comp Mass in 100 g Moles in 100 g N 25.9 1.85 O 74.1 4.63

Determinig Empirical Formula from Percent Composition Data How do you convert the subscripts to small whole numbers? Begin by dividing by the smallest number of moles

Determining Empirical Formula from Percent Composition Data If the result is still not whole numbers, multiply the subscripts by a number that will give you whole number mole ratios

Molecular Formulas Molecular formulas are some whole number multiple of empirical formulas.

Determining Molecular Formulas from Empirical Formulas Molecular formula = n x Empirical formula Usually you will be given the molar mass of the compound Calculate the empirical formula mass Divide molar mass by efm to determine n Then multiply the empirical formula by n to get the molecular formula

Determining Molecular Formula from Empirical Formula Given molar mass = 60.0 g/mol and empirical formula equals CH4N, find the molecular formula. Mol mass = n(empirical formula mass)