Exam # 2 Study Guide Chapters: 3,4,6,7Lecture 1 What is density?The density of an object or substance is the amount of mass ineach standard unit of volume in it. Density tells you how tightlypacked together the matter in the object or substance is.- For example, the density of water is 1.0 g/cm3. That meanseach cm3 of water contains 1.0 g of matter. Two cm3 ofwater contain 2.0 g of matter, three cm3 of water contain3.0 g of matter, and so on.- On the other hand, the density of pure iron is 7.9 g/cm3.That means each cm3 of iron contains 7.9 g of matter. Twocm3 of iron contain 15.8 g of matter, and so forth.Notice the density of iron is much higher than the density of water. That means the matter in iron is packed together more tightly than the matter in water.Density is an intensive property of every object or substance made of matter.An important and useful fact is that the density of a pure substance at a given temperature and pressure is always the same, and the densities of different pure substances are usually different (see table at right). That means we can use the density of a substance to help identify it. For example, if a chemist measures the density of a sample of a clear liquid to be 0.8 g/cm3 at normal temperature and pressure, he can be confident it is not water.What is chemical composition?The chemical composition of a compound is the relative amount of each element in it.- For example, water is a compound made of two elements, hydrogen and oxygen. The chemical composition of water is the relative amounts of hydrogen and oxygen in it.Scientists state chemical composition in two ways:- Mass percent composition. For example, water is 88.8% oxygen and 11.2% hydrogen by mass.- Chemical formula. For example, the chemical formula of water is H2O.One of the most important principles of modern chemistry is that every chemical compound has a unique and unchangeable chemical composition. This is called the Law of Constant Composition.You can convert a mass percent composition into a chemical formula, and vice versa.CHEM 1127Q 1st Editionsubstance density(g·cm−3)air 0.0013gasoline 0.67ice 0.92water 1.0table salt 1.6quartz 2.7iron 7.9gold 19.3Note: the "chemical composition" of a pure substance can also just mean whether the substance is an element, compound or mixture.Lecture 2 (September 9) Why is the division of compounds into ionic and molecular not sharp?What is the ionic character of a compound?When an ionic compound forms from its constituent elements, we usually imagine valence electrons being transferred from one atom to another, forming the cations and anions in each formula unit of the compound:With the transfer of a valence electron, a sodium and chlorine atom form the Na+ and Cl- ions in one formula unit of NaCl.And when a molecular compound forms, we usually imagine atoms sharing valence electrons to form covalent chemical bonds that hold together each molecule of the compound:With the sharing of a pair of valence electrons, two chlorine atoms form one molecule of Cl2.However, it turns out these simple pictures only describe theoretical ideals, and that realchemical compounds are more complicated:- In fact, control of valence electrons is never completely transferred — there's always at least a little bit of sharing, even between the cations and anions in a crystal of an ionic compound.- Control of valence electrons is also rarely shared exactly equally — usually one atom exerts more control over them than the other, making covalent bonds at least a little polar. That is, even in molecular compounds, control of valence electrons is often partially transferred from one atom to another.Chemists accomodate both these facts by saying that real chemical compounds lie on a continuous spectrum between 0% ionic (the theoretically ideal pure molecular compound) and 100% ionic (the theoretically ideal pure ionic compound). Some compounds, like LiF, are very close to 100% ionic, and others, like CH4, are very close to 0% ionic.How do I decide how much ionic character a compound has?Chemists have developed a number of ways to state precisely the amount of ionic character a compound has. For example, for compounds made of diatomicmolecules youcan use the experimental bond dipole moment to calculate a precise percent ionic character.However, often it's enough just to be able to say whether one compound is more ionic than another. For the simplest compounds, this can be done just by comparing where the elements that make up the compound like in the Periodic Table:1. A compound that contains both metals and nonmetals will be much more ionic than a compound that contains only nonmetals.2. If both compounds contain a metal and nonmetal, or two nonmetals, then the most ionic compound is the one in which the elements lie further apart as possible in the Periodic Table.3. When considering distances in the Periodic Table, horizontal distance matters more than vertical distance. Being in a different group matters more thanbeing in a different period.Lecture 3What is a mole?What is the Avogadro Number?The mole is a special SI unit of number used to count atoms and molecules and other exceedingly tiny things.The mole is enormous:1 mol = 6.0221420×1023For example, 1 mole of cabbages is about 6×1023 cabbages, which all together would weigh ten times more than the Moon.Since atoms and molecules are incredibly small, it takes an incredibly big number of them to make up visible amounts of matter. That makes the mole actually a very convenient unit for counting atoms and molecules. For example, an ordinary 8-ounce glass of water contains about 13 moles of water molecules.The mole is defined to be exactly equal to the number of atoms in exactly 12 grams of pure carbon-12. This number is also called the Avogadro Number, and the best modern value ofthe Avogadro Number is 6.0221420×1023.Chemists speak so often about the number of moles of atoms, ions or molecules that they will often leave words like "of atoms", "of ions", or "of molecules" out of their sentences entirely. For example:Chemist: Six grams of carbon is about half a mole.Translation: Six grams of carbon is about half a mole of carbon atoms.Chemist:Translation: Each mole of NaCl produces a mole of Na+ when it dissolves.Each mole of formula units of NaCl produces a mole of Na+ ions when it dissolves.Chemist:Translation:There are fewer moles of glucose than of fructose.There are fewer moles of molecules of glucose than of