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Molecular Compounds

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University of Toronto Scarborough
Maydianne Andrade

Molecular Compounds A molecular compound is made up of discrete units called molecules, which typically consist of a small number of non-metal atoms held together by covalent bonds. Molecular compounds are represented by chemical formulas, symbolic representations that, at minimum, indicate: the elements present and the relative number of atoms The 2ch element. elements Example: H 2 Two H Lack of subscript means 1 atoms of O per atoms per molecule Empirical Formula The empirical formula is the simplest formula for a compound where its subscripts are reduced to their simplest whole number ratios. Generally, the empirical formula does not tell one a significant about of information about a given compound. Acetic acid (2 4 2 ) and glucose 6C12 6 ) both have the empirical formul2 CH O. Molecular Formula The molecular formula is based on an actual molecule of a compound. In some cases, the empirical and molecular formulas are identical, such as formaldehyde (CH2O). In other cases, the molecular formula is a multiple of the empirical formula. Both the empirical and molecular formulas provide information on the combining rations of the atoms in the compounds, however, they show nothing on how the atoms are attached relative to each other. Structural Formula The structural formula shows the order in which atoms are bonded together in a molecule and by what types of bonds. For example acetic acid: H O H C C O H H The covalent bonds in the structural formula are represented by lines or dashes and a double bond is represented by a double dash. Condensed Structural Formula This formula is a representation of the molecular structure of a compound which is written in one line and is an alternate way of showing how the atoms of a molecule are connected. For example with acetic acid: CH COOH or CH CO H. 3 3 2 It can also be used to show how a group of atoms is attached to another atom. For example, methylpropane (C 4 10 CH 3 CH C3 CH 3 can be written as CH 3H (CH ) C33 or asCH (CH ) 3 3 CH 3 The parentheses indicate that that ligand is connected to the molecule at that given point. Line angle Formula The line angle formula contains lines that represent chemical bonds. For example, the male hormone molecule, testosterone. OH H3C H 3 O Ionic Compounds An ionic compound is made up of positive and negative ions joined together by electrostatic forces of attraction. The atoms of metallic elements tend to lose one or more electrons when combining with non-metal atoms. The non-metal atoms, in return, gains one or more electrons in the process. When the metal loses an electron during transfer, it becomes a cation (positive ion) and the non-metal atom becomes an anion (negative ion). + Consider NaCl. An ionic crystal of NaCl shows that one Na ion is surrounded by six Cl ions. We cannot pick any one of these Cl ions and exclusively associate it with a + given Na , however, since the ratio of chloride ions to sodium ions is 1:1, and so a combination of one sodium ion and one chloride ion as a formula unit. The formula unit of an ionic compound is the smallest electrically neutral collection of ions. The ratio of atoms in the formula unit is essentially the same as in the chemical formula. Since a formula unit is only a fragment of a vast network of ion (crystal), a formula unit cannot be distinguished as distinct. Therefore, it is incorrect to label a formula unit as a molecule. Ions such as Na , Mg 2+ and Cl are monatomic, meaning that each consists of a single ionized atom. Alternately, and ion made up of two or more atoms are called polyatomic ions (e.g. NO ). (St3dy polyatomic ions) The Mole Concept and Chemical Compounds Formula mass: the mass of a formula unit in atomic mass units. It is appropriate to use the term formula mass, but for molecular compounds, the formula unit is actually a molecule, so the term molecular mass is used. The molecular mass is the mass of a molecule in atomic mass units (amu). Mole of a Compound The molar mass is the mass of one mole of compound – one mole of molecules of a molecular compound and one mole of formula units of an ionic compound. Problem: An analytical balance can detect a mass of 0.1 mg. What is the total number of ions present in this minimally detectable quantity of MgCl ? 2 Solution: Use molar mass to convert from mass to number of moles of MgCl . Then 2 use Avogadro constant as a conversion factor to convert moles to number of formula units (fu). Finally, there are three ions (1 Mg 2+and two Cl ) per formula unit of magnesium chloride. ? ions = [0.1mg MgCl x (2 g MgCl / 1002 mg MgCl ) x (1 mol2MgCl / 95 g MgCl )2x (6.0 x 2 10²³ fu MgCl /21 mol MgCl ) x 23 ions / 1 fu MgCl )] 2 18 = 2 x 10 ions Problem: The volatile liquid ethyl mercaptan, C H S, i2 o6e of the most odoriferous substances known. It is sometimes added to natural gas to make gas leaks detectable. How many C H S molecules are contained in a 1.0 µL sample? (d=0.84 2 6 g/mL) Solution: ? molecules C H 2 =6 -6 [1.0 µL x (1 x 10 / 1µL) x (1000mL / 1L) x (8.4 g C H S / 1 m2) 6 (1 mol C H S / 62.1 2 6 g C 2 S6 x (6.022 x 10²³ molecule C H S / 12m6l C H S] 2 6 18 = 8.1 x 10 molecules C H S2 6 Some Familiar Molecular Formulas H , O , N , F , Cl , Br , I , P , S 2 2 2 2 2 2 2 4 8 Composition of Chemical Compounds Consider Halothane C HBrCl2 3 Mole ratio: nc/ n halothane Mass ratio: m /cm halothane M(C H2rClF )= 3M + M +CM H BR + M +ClM F =(2 x 12.01) + 1.01 + 79.90 + 35.45 + (3 X 19.00) = 197.38 g / mol Problem: How many moles of F atoms are in a 75.0 mL sample of halothane (d = 1.871 g / mL)? Solution: ? mol F = 75.0 mL C HBr2lF x (1.371g C HBrClF /21 mL C HB3ClF ) x (12mol C HB3ClF / 2 3 197.4g C HBrClF ) x (3 mol F/ 1 mol C HBrClF ) 2 3 2 3 Calculating Percent Composition From a Chemical Formula 1. Determine the molar mass of the compound. This is the denominator in the equation. 2. Determine the contribution of the given element to the molar mass. This product of the formula subscript and the molar mass of the element appears in the numerator of the equation. 3. Formulate the ratio of the mass of the given element t
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