Chapter 8 Chemical Composition

8.1 Counting by Weight  It is often easier to determine the number of items in a sample by weighing, not by counting. Many small candies are sold by mass for this reason.  It is important to note that not every jellybean has the same mass, but on average the mass is a constant number.

 The mass of an atom is a weighted mass, called average atomic mass. Every substance has a different average atomic mass.  Two different substances contain the same number of components if the ratio of the sample masses is the same as the ratio of the masses of the individual components.

8.2 Atomic Masses: Counting Atoms by Weighing  Atoms are too small to see, so atoms are often counted by weighing a sample containing a large number of atoms.  The atomic mass unit is used to weigh atoms because normal units of mass are simply too large.  The atomic mass unit (amu) is equal to 1.66 x g.

 The average atomic mass is a weighted average of the various isotopes of an element.  Using average atomic mass we can treat any sample of an element as if it is composed of identical atoms each with a mass equivalent of the average atomic mass in amu.

Examples 1. Calculate the mass, in amu, of a sample of aluminum that contains 75 atoms. 2. Calculate the mass of a sample that contains 23 nitrogen atoms. 3. Calculate the number of sodium atoms present in a sample that has a mass of amu. 4. Calculate the number of oxygen atoms in a sample that has a mass of 288 amu.

8.3 The Mole  The atomic mass unit is a very small unit of mass; a unit that is not convenient in laboratory use. Scientists use grams when working in a laboratory.  Samples in which the ratio of the masses is the same as the ratio of the masses of the individual atoms always contain the same number of atoms.

 In chemistry if the weight of a sample of an element has a mass equal to that element’s average atomic mass in grams, the sample contains the same number of atoms.  This number of atoms is called the mole.

 The number of atoms in a mole is called Avogadro’s number, and is equal to x  One mole of anything consists of x  A dozen means twelve no matter what is being referred to; a mole means x no matter what is being referred to.  A sample of an element with a mass equal to that elements average atomic mass expressed in grams contains 1 mole of atoms.

Examples 1. Aluminum, a metal with a high strength to weight ratio and a high resistance to corrosion is often used for structures, such as bicycle frames. Compute both the number of moles of atoms and the number of atoms in a 10.0 g sample of aluminum. 2. 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? 3. Chromium is a metal that is added to steel to improve its resistance to corrosion. Calculate both the number of moles in a sample of chromium containing 5.00 x atoms and the mass of the sample. 4. Calculate the number of atoms in a 23.6 mg sample of zinc. 5. Calculate the number of atoms in a mg sample of silver.

8.4 Molar Mass  A chemical compound is a collection of atoms.  To calculate the mass of one mole of a compound you sum the masses of each of the atoms present.  The mass of a mole of a compound in grams is called the molar mass.

Examples 1. Calculate the molar mass of sulfur dioxide, a gas produced when sulfur containing fuels are burned. 2. Polyvinyl chloride (PVC), which is used for floor coverings and plastic pipes is made from a molecule with the formula C2H3Cl. Calculate the molar mass of this substance. 3. Calculate the molar mass of the following. Water Ammonia (NH 3 ) Ammonia (NH 3 ) Propane (C 3 H 8 ) Propane (C 3 H 8 ) Glucose (C 6 H 12 O 6 ) Glucose (C 6 H 12 O 6 ) 4. Calcium carbonate is the principal mineral found in limestone and the shells of marine animals. Calculate the molar mass of calcium carbonate Calculate the molar mass of calcium carbonate A certain sample of calcium carbonate contains 4.86 moles. What is the mass in grams of this sample? A certain sample of calcium carbonate contains 4.86 moles. What is the mass in grams of this sample?

 Often the molar mass of an ionic compound is referred to as a formula weight, but the term molar mass can be used for both ionic and covalent compounds.  The molar mass of a substance can be obtained by summing the masses of the component atoms.  The molar mass in grams represents the mass of one mole of a substance.  Once the molar mass is known the number of moles present in a sample of known mass can be determined.

8.5 Percent Composition of Compounds  The composition of a compound in terms of the relative masses of its constituent elements is often used by chemists in the laboratory.  To determine the composition of a compound by mass you compare the mass of each element present in one mole of the compound to the total mass of the one mole of the compound.

Examples 1. Determine the pass percent of ethanol, C 2 H 5 OH. 2. Carvone is a substance that occurs in two forms, both of which have the same molecular formula (C 4 H 10 O). Compute the mass percent of each element in carvone. 3. Penicillin, an important antibiotic, was discovered by Alexander Fleming in One type of penicillim, penicillin F has the molecular formula C 14 H 20 N 2 SO 4. Compute the mass percent of each element in this compound.

8.6 Formulas of Compounds  To obtain the formula of an unknown compound chemists use the measured masses of the elements present to determine the formula.  When chemists break a compound down into its separate elements and count the atoms present, they learn only the ratio of the atoms, that is they get only the relative number of atoms.

 An empirical formula is the formula of a compound that represents the smallest whole number ratio of the atoms present.  The actual formula of the compound, the formula that gives the composition of the molecules that are present is called the molecular formula.

Examples 1. In each case below, the molecular formula for a compound is given. Determine the empirical formula for each compound. C 6 H 6 C 6 H 6 C 12 H 4 Cl 4 O 2 C 12 H 4 Cl 4 O 2 C 6 H 16 N 2 C 6 H 16 N 2 2. Determine the empirical formula from each of the following molecular formulas. H 2 O 2 H 2 O 2 C 4 H 10 C 4 H 10 CCl 4 CCl 4

8.7 Calculation of Empirical Formulas  To calculate the empirical formula of a compound, determine the relative masses of the various elements that are present, and then convert the masses to moles.

Examples 1. An oxide of aluminum is formed by the reaction of g of aluminum with g of oxygen. Calculate the empirical formula of this compound. 2. When a g sample of vanadium metal is heated in air, it reacts with oxygen to achieve a final mass of g. Calculate the empirical formula of this vanadium oxide. 3. A sample of lead arsenate, an insecticide used against the potato beetle, contains g of lead, g of hydrogen, g of arsenic, and g of oxygen. Calculate the empirical formula for lead arsenate. 4. Cisplatin, the common name for a platinum compound that is used to treat cancerous tumors, has the composition 65.02% platinum, 9.34% nitrogen, 2.02% hydrogen, and 23.63% chlorine. Calculate the empirical formula for cisplatin.

8.8 Calculation of Molecular Formulas  In order to calculate the molecular formula of a compound the composition (in terms of masses or mass percentages) and the molar mass must be known.

Examples 1. A white powder is analyzed and found to have an empirical formula of P 2 O 5. The compound has a molar mass of g. What is the compounds molecular formula? 2. A compound containing carbon, hydrogen, and oxygen is found to be 40.00% carbon and 6.700% hydrogen by mass. The molar mass of the compound is between 115 g/mol and 125 g/mol. Determine the molecular formula for this compound. 3. Caffeine is a compound containing carbon, hydrogen, nitrogen, and oxygen. The mass percent composition of caffeine is 49.47% carbon, 5.191% hydrogen, % nitrogen, and 16.48% oxygen. The molar mass of caffeine is approximately 194 g/mol. Determine the molecular formula for caffeine.