Chemistry Chapter 3 (Part I) Stoichiometry

Section 1 Counting by Weighing

Chemical Stoichiometry Quantities of materials consumed and produced in chemical reactions.

Counting by Weighing Atoms are so small/interact with matter that contain huge numbers of atoms Not possible to count them directly Determine the number of atoms in a given sample by finding its mass Must know the average mass of the atoms

Section 2 Atomic Masses

Atomic Mass Carbon-12 is the standard Carbon-12 is assigned a mass of exactly 12 amu and the masses of all other atoms are given relative to this standard

Atomic Mass Mass Spectrometer Atoms are passed into a beam of high speed electrons Applied electric field then accelerates these ions into a magnetic field An interaction occurs, which changes the path of the ion The amount of path deflection for the ion depends on its mass The most massive ions are deflected the smallest/ratio to carbon-12

Atomic Mass Average atomic mass Reflect the average of the isotopes Example: carbon is 12.01 amu Mass spectrometer can also give percent abundance

Calculation of average atomic mass A sample of metal is vaporized and injected into a mass spectrometer. The mass spectrometer tells us that 60.10% of the metal is present as 68.93 amu and 39.90% of the metal is present as 70.92 amu. What is the average atomic mass of the element? What is the identity of the element? p. 85

Section 3 The Mole

The Mole Mole The number equal to the number of carbon atoms in exactly 12 grams of pure carbon-12 Avogadro’s number = 6.022 x 1023 units 1 mole = Avogadro's number The mass of 1 mole of an element is equal to its atomic mass in grams

Conversion between Atoms, Moles, and Mass Compute both the number of moles of atoms and the number of atoms in a 10.0 g sample of aluminum. A silicon chip used in an integrated circuit of a microcomputer has a mass of 5.68 mg. How many silicon atoms are present in this chip? Calculate both the number of moles in a sample of cobalt containing 5.00 x 1020 atoms and the mass of the sample. p. 85

Section 4 Molar Mass

Molar Mass The mass in grams of one mole of the compound Unit = g/mol

Calculating Molar Mass Juglone, a dye known for centuries, is produced from the husks of black walnut. It is also a natural herbicide that kills off competitive plants around the black walnut tree but does not affect grass and other noncompetitive plants. The formula for juglone is C10H6O3. Calculate the molar mass. A sample of 1.56 x 10 -2 g of juglone was extracted from black walnut husks. How many moles of juglone does this sample represent? p. 85

Calculating Molar Mass Calcium carbonate (CaCO3) is the principal mineral found in limestone, marble, chalk, pearls, and the shells of marine animals. Calculate the molar mass. A certain sample of calcium carbonate contains 4.86 moles. What is the mass in grams of this sample? What is the mass of the carbonate ions present? p. 85

Calculating Molar Mass 3. Isopentyl acetate (C7H14O2) is the compound responsible for the scent of bananas. Interestingly, bees release anout 1 microgram of this compound when they sting. The resulting scent attracts other bees to join the attack. How many molecules of isopentyl acetate are released in the typical bee sting? How many atoms of carbon are present? p. 85

Homework Ch 3 Review Page 118 #33, 37, 41, 43, 47, 49

Percent Composition of Compounds Section 5 Percent Composition of Compounds

Percent Composition Two common ways of describing the composition of a compound: Terms of numbers of its constituent atoms Terms of percentages of its elements

Percent Composition Penicillin, the first of a now large number of antibiotics, was discovered accidently by Alexander Fleming in 1928, but was never able to isolate its pure compound. Penicillin F has the formula C14H20N2SO4. Compute the mass percent of each element. p. 85

Determining the Formula of a Compound Section 6 Determining the Formula of a Compound

Determining the Formula of a Compound Two common ways of determining the formula of a compound: Decomposing it into its component elements

Determining the Formula of a Compound Two common ways of determining the formula of a compound: Reacting it with oxygen to produce substances such as carbon dioxide, water, or nitrogen Calculate the fraction by mass of each element in the compound

Empirical Formula Determination Empirical Formula: The simplest whole number ratio of atoms in a compound.

Empirical Formula Determination Since mass percentage gives the number of grams of a particular element per 100 grams of compound, base the calculation on 100 grams of a compound. Each percent will then represent the grams of that element.

Empirical Formula Determination Determine the number of moles of each element. Divide each value of the number of moles by the smallest of the values. If .5, multiply all by 2 If .3, multiply all by 3 If .25, multiply all by 4

Molecular Formula Determination Molecular Formula: The exact formula of a molecule, giving the types of atoms and the number of each type.

Molecular Formula Determination Obtain the empirical formula. Compute the molar mass of the empirical formula. Calculate the ratio: n= Molecular Formula Molar Mass Empirical Formula Molar Mass

Molecular Formula Determination n(Empirical formula)=Molecular Formula

Molecular Formula Determination Determine the empirical formula and molecular formula for a compound that gives the following percentages upon analysis: 71.65% chlorine 24.27% carbon 4.07% hydrogen The molar mass is known to be 98.96 g/mol.

Molecular Formula Determination Caffeine contains 49.48% carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen by mass and has a molar mass of 194.2 g/mol. Determine the molecular formula of caffeine.

Homework Chapter 3 (Part I) Review worksheet