Molar Mass. The total of the atomic masses of all the atoms in a molecule:

Transcription

1 Molar Mass The total of the atomic masses of all the atoms in a molecule: Ex: H 2 O H (1.0079) x 2 atoms = grams O (15.999) x 1 atom = grams grams (18.0 grams) Ex: Cu(NO 3 ) 2 Cu (63.546) x 1 atom = grams N (14.007) x 2 atoms = grams O (15.999) x 6 atoms = grams grams (187.6 grams) Atoms- small quantities - difficult to see and count * Chemists use mole counting unit (other common counting units: pair, dozen, ream, etc ) * 1 mole of anything (atoms, molecules, etc ) is equal to: This is called Avogadro s number 6.02 x Ex: How many atoms are in 1 mole of Carbon? 1 mole of Carbon = 6.02 x atoms (or molecules) Also, 1 mole of any element = the mass of that element Ex: 1 mole of Carbon (C) - find the mass and round to tenths place 1 mole of C = = 12.0 grams Ex: 1 mole of Copper (Cu): 1 mole of Cu= = 63.6 grams Finding the molar mass of a COMPOUND: sum of molar masses of atoms CuO Cu 63.5 grams grams Total 79.5 grams CuO 1 mole of CuO is 79.5 grams

2 Calculating % Composition I. From the chemical formula: Calculate the formula mass Divide the atomic mass of each element by the formula mass of the compound Multiply by 100 Ex: H 2 O H (1.0079) x 2 = grams O (15.999) = grams Formula Mass = grams = 18.0 grams % H = 2.02 x 100 = 11.2 % H 18.0 % O = 16.0 x 100 = 88.8 % O 18.0 II. From masses of each element: Divide the mass of each element by the total mass of the compound Multiply by 100 Ex: 55 grams of element X 45 grams of element Y 100 grams total 55 grams x 100 = 55 % element X 100 grams 45 grams x 100 = 45 % element Y 100 grams

3 Converting using Mass, Moles, and Molecules Basic steps: 1. Always start with what is given 2. Use known conversion factors (1 mole = 6.02 x ) or (1 mole = atomic mass) 3. Cancel your units 4. Multiply and divide Moles Mass Problems (converting moles to grams) Ex: If you have 2 moles of Carbon, how many grams is that? 2 moles C x 12.0 grams C = 24.0 grams Carbon 1 mole C Moles Molecules Problems (converting moles to molecules or atoms) Ex: If you have 2 moles of Carbon, how many molecules is that? 2 moles C x 6.02 x molecules = x molecules 1 mole C Mass Molecules Problems (converting grams to molecules or atoms) Ex: If you have 30 grams of Carbon, how many atoms is that? 30 g C x 1 mole C x 6.02 x atoms = atoms 12.0 g C 1 mole C

4 Empirical Formula Use when you know which elements are in the formula, and in what percentage but you don t know the molecular formula Ex: % Sulfur % Oxygen 100 % total Equate each % to grams: Sulfur: Oxygen: Total: grams grams grams Convert from grams to moles: grams S x 1 mole S = 1.25 mole S g S grams O x 1 mole O = 3.75 mole O 16.0 g O The mole ratio is 1.25: 3.75 (S:O), however, these are not whole numbers so they can t be subscripts. To get them into whole numbers: Divide both mole values by the lowest number: 1.25 mole S = 1 mole S 1.25 mole S 3.75 mole O = 3 mole O 1.25 mole O (** If still not a whole number, multiply each by 2, 3, 4.. until it is a whole number) Therefore, the formula is SO 3

5 Interpreting Equations in terms of Moles and Mass 4 Fe + 3 O 2 2 Fe 2 O 3 You can interpret any balanced equation in terms of moles, mass, and molecules Molecules: 4 atoms of iron react with 6 atoms of oxygen to produce 2 molecules of iron (III) oxide Moles: 4 moles of iron react with 3 moles of oxygen to produce 2 moles of iron (III) oxide Mass: Convert to grams first Reactants: 4 mole Fe x g Fe = g Fe 1 mole Fe Product: 3 mole O 2 x g O 2 = 96.0 g O 2 1 mole O 2 2 mole Fe 2 O 3 x g Fe 2 O 3 = g Fe 2 O 3 1 mole Fe 2 O 3 ** Law of Conservation of Mass the sum of the reactants should ALWAYS equal the sum of the product check your work!!! grams of Fe react with 96.0 grams of O 2 to produce grams of Iron (III) oxide Mole Ratios Coefficients indicate the number of moles Mole Ratios the ratio between the numbers of moles of any 2 substances in a balanced equation Ex: 2 Al + 3 Br 2 2 AlBr 3 All possible mole ratios: 2 mole Al : 3 mole Br 2 2 mole Al : 2 mole AlBr 3 3 mole Br 2 : 2 mole Al 3 mole Br 2 : 2 mole AlBr 3 2 mole AlBr 3 : 2 mole Al 2 mole AlBr 3 : 3 mole Br 2

6 Stoichiometry Calculations I. Mole to mole conversions: Ex: Determine the number of moles of H 2 produced when moles of K is used. Steps: 1. Write the balanced equation: 2 K + 2 H 2 O 2 KOH + H 2 2. Identify the known (given) substances and the unknown substances: Known:.040 moles K Unknown: moles of H 2 3. Determine the mole ratio of unknown substance to known substance: 1 mole H 2 (use coefficients from balanced equation) 2 mole K 4. Convert using the known substance and the mole ratio: moles K x 1 mole H 2 = 0.02 mole H 2 produced 1 2 mole K II. Mole to mass conversions: Ex: Determine the mass of NaCl produced when 1.25 moles of Cl 2 reacts with sodium Steps: 1. Write the balanced equation: 2 Na + Cl 2 2 NaCl 2. Identify the known (given) substances and the unknown substances: Known: 1.25 moles Cl 2 Unknown: mass of NaCl 3. Determine the mole ratio of unknown substance to known substance: 2 moles NaCl (use coefficients from balanced equation) 1 mole Cl 2 4. Convert using the known substance and the mole ratio: 1.25 moles Cl 2 x 2 moles NaCl = 2.5 moles NaCl 1 1 mole Cl 2 5. Convert moles of unknown to grams of unknown: (use molar mass) 2.5 mole NaCl x g = 146 grams NaCl 1 1 mole

7 III. Mass to mass conversions: Ex: Determine the mass of H 2 O produced from the decomposition of 25.0 grams of NH 4 NO 3 Steps: 1. Write the balanced equation: NH 4 NO 3 N 2 O + 2H 2 O 2. Identify the known (given) substances and the unknown substances: Known: 25.0 grams of NH 4 NO 3 Unknown: mass of H 2 O 3. Convert grams of known to moles of known: 25.0 g NH 4 NO 3 x 1 mole NH 4 NO 3 = mole NH 4 NO g 4. Determine the mole ratio of unknown substance to known substance: 2 moles H 2 O (use coefficients from balanced equation) 1 mole NH 4 NO 3 5. Convert using the known substance and the mole ratio: mole NH 4 NO 3 x 2 moles H 2 O = mole H 2 O 1 1 mole NH 4 NO 3 6. Convert moles of unknown to grams of unknown: (use molar mass) mole H 2 O x 18.0 g = 11.2 grams H 2 O 1 1 mole H 2 O