Chemical Quantities

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 representative 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

Challenge • What is the mass of 2.8 x 1024 molecules of SO2?

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.

Mole Road Map

Molar Mass & Density of Gases; Percent Composition: Objectives • By the end of this lesson we should be able to • Calculate density of a gas at STP • Determine molar mass of a gas at STP from density • Determine % composition of a substance

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

Molar Mass & Density of Gases • 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: Practice • 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.

Molar Mass & Density of Gases: Formative Assessment • Find the density of a gas with a molar mass of 80.0 g/mol.

Molar Mass & Density of Gases: Formative Assessment

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

How to Determine % Composition • % comp is the mass of an element within a compound divided by the molar mass of the compound • Example: ammonia, NH3 • %N in NH3 = mass of N/molar mass of NH3 • %N = 14.01 g/17.04 g =0.822 = 82.2% • %H = 3(1.01) g/17.04 g = 0.178 = 17.8% or…. • %H = 1-0.882 = 0.178 = 17.8%

Percent Composition: Formative assessment • Determine % composition of glucose, C6H12O6 • Find the % chromium in potassium dichromate, K2Cr2O7

Percent Composition: Formative assessment • Determine % composition of glucose, C6H12O6 • MM = 180.18 g/mol • %C = 12.01(6)/180.18 = 0.40 = 40% • %H = 1.01(12)/180.18 = 0.0673 = 6.73% • %O = 16(6)/180.18 = 0.5328 = 53.28%

Percent Composition: Formative assessment • Find the % chromium in potassium dichromate, K2Cr2O7 • MM = 294.2 g/mol • % Cr = 52.0(2)/294.2 = 0.353 = 35.3% by mass

Percent Composition and Empirical Formulas • By the end of this lesson you will be able to • Explain what an empirical formula is • Determine an empirical formula from % composition data

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 ?????

Determining 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

Determining Molecular Formulas • By the end of this lesson you will be able to • Explain the difference between an empirical formula and a molecular formula • Use an empirical formula and molar mass to determine the molecular formula of a substance

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 (EFM) • 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)

Chemical Quantities