MOLE AND MATH
THE MOLE (a counting unit) A mole represents a set or group, much in the same way that a dozen represents a set of twelve. 1 dozen eggs = 12 eggs; 1 mole eggs = 6.022 x 10 23 eggs 1 dozen carbon atoms = 12 carbon atoms; 1 moel carbon atoms = 6.022 x 10 23 carbon atoms The number 6.022 x 10 23 is known as Avogadro s Number.
THE MOLE (a counting unit) The number 6.022 x 10 23 is known as Avogadro s Number. If you write out Avogadro s number it would look like this: 602,200,000,000,000,000,000,000
THE MOLE (a counting unit) Remember: 1 mole of atoms = 6.022 x 10 23 atoms 1 mole of molecules = 6.022 x 10 23 molecules 1 mole of formula units = 6.022 x 20 23 formula units 1 mole of ions = 6.022 x 10 23 ions
Atomic mass vs. Molar mass Atomic mass = the mass of ONE ATOM expressed in atomic mass units (amu) Ex: Oxygen s atomic mass = 15.999 amu; this is the average mass of 1 oxygen atom Molar mass = the mass of ONE MOLE of atoms, molecules, or formula units expressed in grams Ex: Oxygen s molar mass = 15.999 grams; this is the mass of 6.022 x 10 23 oxygen atoms
THE MOLE (a counting unit) Element and Symbol Atomic Mass Average mass of 1 atom Molar mass mass of 1 mole of atoms (6.022 x 10 23 atoms) Carbon (C) 12.0 amu 12.0 g C Helium (He) 4.00 amu 4.00 g He
CALCULATING MOLAR MASS
I. Calculating Molar Mass 1. Phosphoric acid H 3 PO 4
I. Calculating Molar Mass 2. Calcium Nitrate -
I. Calculating Molar Mass 3. Iron (III) Carbonate -
Calculating Percent Composition
II. Finding Percent Composition Percent Composition = is the percent by mass of each element in the compound. The percent composition is always the same regardless of the size of the sample. Ex: H 2 O is ALWAYS 11.2% hydrogen and 88.8% oxygen (by mass)
II. Finding Percent Composition The percent composition of the elements in a compound adds up to 100%
II. Finding Percent Composition Formula Used: Molar mass of the element in a compound x 100 Total molar mass of the compound Or this of it as: Part x 100 Whole
II. Finding Percent Composition Steps: 1. Find molar mass of each element. 2. Find the total molar mass of the compound (whole) 3. Part x 100 Whole 4. Evaluate your answer (percentages should add to approximately 100%)
II. Finding Percent Composition Ex: Find the percent composition of aluminum sulfate.
II. Finding Percent Composition NOTE: Therefore, a 100g sample of Al 2 (SO 4 ) 3 would contain 16 g Al, 28 g S, and 56 g O.
MOLAR CONVERSIONS
III. CONVERSIONS BETWEEN GRAMS AND MOLES KEY IDEA: It is important to know the following conversion: The molar mass of a compound or the mass of an element is equal to 1 mole of that compound or element. Ex: 1 mole of carbon is equal to 12 grams of carbon Ex: 1 mole of water is equal to 18 grams (molar mass) of water.
III. CONVERSIONS BETWEEN GRAMS AND MOLES Use a T-chart to convert from what you know to what you do not know. Use conversion factors so units cancel out!
III. CONVERSIONS BETWEEN GRAMS AND MOLES Ex #1: How many moles are represented by 11.5 g of magnesium hydroxide?
III. CONVERSIONS BETWEEN GRAMS AND MOLES Ex #2: How many grams of aluminum sulfide are present in 0.44 moles of aluminum sulfide?
CALCULATING FORMULAS
IV. CALCULATION OF FORMULAS: 2 types of formulas: 1. Molecular formula (M.F.) = shows the types and numbers of atoms combined in a single molecule. Ex: C 6 H 12 O 6 (glucose) Actually has 24 atoms!
IV. CALCULATION OF FORMULAS: 2. Empirical formula (E. F.) = a chemical formula showing the simplest whole number ratio of atoms in a compound. Ex: Reduce glucose C 6 H 12 O 6 by dividing all of the subscripts by 6 the empirical formula would be CH 2 O
IV. CALCULATION OF FORMULAS: In the chart below, fill in the empirical formula given the molecular formula. Molecular Formula Empirical Formla H 2 O 2 H 2 O C 8 H 16 C 2 H 4
IV. CALCULATION OF FORMULAS: Notice 2 things: 1. Some molecular formulas cant be reduced (H 2 O). 2. Several compounds share the same E.F., but each compound has its how unique molecular formula. Molecular Formula H 2 O 2 H 2 O Empirical Formula HO H 2 O C 8 H 16 CH 2 C 2 H 4 CH 2
IV. CALCULATION OF FORMULAS: WHY DO WE USE THE EMPIRICAL FORMULA IF IT CONTAINS LESS INFORMATION? It is easier to experimentally determine the empirical formula compound than the molecular formula. So the E.F. is usually found first as a step towards finding the molecular formula.
Calculating Empirical Formulas
IV. CALCULATION OF FORMULAS: Steps to finding an EMPIRICAL FORMULA: 1. The amount of each element should be listed in grams. If analysis of the compound is in percent, convert to grams: assume a 100 g sample of the compound. 2. Convert grams of EACH element to moles (T-chart). 3. Divide moles of each element by the smallest number of moles. 4. Round each answer from 3 to the nearest whole number and write the empirical formula using these numbers as subscripts.
EMPIRICAL FORMULAS: Ex #1: What is the empirical formula for a compound that contains 0.9g of calcium and 1.6g of chlorine?
EMPIRICAL FORMULAS Ex #2: What is the empirical formula for a compound that is 40% C, 6.71% H, and 53.3% O?
Calculating Molecular Formulas
Steps to finding a MOLECULAR FORMULA 1. If it is not given, determine the empirical formula of the compound. (Follow all steps necessary to determine an empirical formula). 2. Calculate molar mass of the empirical formula. 3. Divide the molar mass of the compound by the molar mass determined from the empirical formula. 4. Round the answer to 3 to the nearest whole number and multiply all subscripts in the empirical formula by this number.
MOLECULAR FORMULAS Ex #1: What is the molecular formula of a substance that has an empirical formula of AgCO 2 and a molar mass of 304g/mol?
MOLECULAR FORMULAS Ex #2: An unknown compound contains 85.64% carbon and 14.36% hydrogen. It has a molar mass of 42.08 g/mol. Find its molecular formula.