Chapter 3: Chemical formulas and Composition Stoichiometry Understand early concepts of atoms Use chemical formulas to solve various kinds of chemical problems Recognize and use formular mass (weight) Interconvert masses, moles and formulas Determine formulas from composition

New vocabularyStoichiometry: describes the quantitative relationships among elements in compounds (composition stoichiometry)

Chemical formulasThe chemical formulas for a substance shows its chemical compositionThe formula for a single atom is the same as the symbol for the element.Chemical formula gives the number of atoms of each type in the moleculeStructural formula shows the order in which atoms are connected

Chemical & structural formulas Ball and stick model

Names and chemical formulas of some common molecular compoundsH2OH2O2HCl * (gas)H2SO4HNO3CH3COOHNH3

SO2SO3COCO2CH4C2H6C3H8* Called hydrochloric acid if dissolved in water

Ions and ionic compoundsAn ion is an atom or a group of atoms that carries an electric charge. Ions that posses a positive charge are called cations. Those carrying a negative charge are called anions. Ions are formed when neutral atoms lose or gain electrons

Example about ionic substanceThe arrangement of ions in NaCl crystal6For ionic substances, there are no molecules

Instead, we refer to formula unit Compound NaCl consists of collections of largenumbers of ions

Formulas, ionic charges and names of some common ionsCommon cationsNa+SodiumK+PotassiumNH4+Ag+SilverMg2+Ca2+CalciumZn2+ZincCu+Copper(I)Cu2+Copper(II)Fe2+Iron(II)Fe3+Iron (III)Al3+AluminumCommon anionsF-fluorideCl-chlorideBr-bromideOH-CH3COO-acetateNO3-nitrateO2-S2-sulfideSO42-sulfateSO32-sulfiteCO32-carbonatePO43-phosphate

Those in red color are polyatomic ions (group of atoms that bear an electric charge)

Names and formulas of some ionic compounds

All group B metals with the exception of silver (which is always found as Ag+) and zinc (always Zn2+) can take on a variety of chargesA Roman numeral placed within parentheses after the name of the cation gives the positive charge of that cation.

Common Polyatomic Ions

Some examples of ionic compoundsTable saltCalcium fluoride

ExampleName the following ionic compounds:(NH4)2S; Cu(NO3)2; ZnCl2; Fe2(CO3)3Write the formulas for the following ionic compounds:Sodium fluoride; calcium fluoride; iron(II) sulfate; zinc phosphate

Prefixes for Binary Molecular CompoundsCompounds made up of nonmetals held together by one or more covalent bonds are molecular compounds

Example

Atomic mass (traditionally called atomic weight-AW)Atomic mass unit (amu) is defined as exactly 1/12 of the mass of an atom of a particular kind of carbon atom, called carbon-12.On this scale, the atomic weight of H is about 1 amu (1.00794 amu), of sodium (Na) is 22.989768 amu.

The mole (mol)1 mole = 6.023 x 1023 particles (atoms, molecules or other)The mass of one mole of atoms of a pure element in gram is numerically equal to the atomic weight of that element in atomic mass unit. This is also called the molar mass of the element; its unit are grams/mole (g/mol)

ExampleHow many moles of atoms does 136.9 g of iron metal contain (know that atomic weight of iron is 55.85 amu)How many atoms are contained in 2.451 mol of iron (III) oxide

Formula weights, molecular weights and molesThe formula weight (FW) of a substance is the sum of the atomic weights (AW) of the elements in the formula. Therefore, a formula weight gives the mass of one formula unit in atomic mass unit (amu)Molecular weight (MW) is used instead formula weight when we refer specifically to molecular (non-ionic) substances

ExerciseCalculate the formula weight of acetic acid (vinegar), CH3COOH (Know that atomic weight (AW) of C, H, O are 12, 1 and 2 amu, respectively.How may (a) moles of O2, (b) O2 molecules, and (c) O atoms are contained in 40.0 g of oxygen gas (O2) (Know that AW of O is 16 amu)

Percent composition (by mass) and formulas of compoundsIf the compounds formula is known, its chemical composition can be expressed as the mass percent of each element in the compound (percent composition).Percentage is the part divided by the whole multiplies 100 percent (simply parts per 100)Calculate the percent composition by mass of HNO3 (Know that AW of H, N and O are 1, 14.0 and 16.0, respectively.

Simplest (emperical) formulasThe simplest formula for a compound is the smallest whole-number ratio of atoms present.Example: HO is the simplest formula of H2O2

Example

ExerciseAnalysis of a sample of a pure compound reveals that it contains 50.1% sulfur and 49.9% oxygen by mass. What is the simplest formula of the compound? (Know that AW of O and S are 16 and 32.1 amu, respectively).

Moving from Empirical Formulas to Molecular Formulas

W = mg In which, W: weight of objectm: mass of objectg: acceleration of gravityg on the earths surface=9.8 m/s2

IsotopesHydrogenDeuterium (D)Tritium (T)

Exercise

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