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# UMass Amherst CHEM 110 - Chemistry Chapter 3

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Chemistry Chapter 3Part 1● Stoichiometry: The relationship between quantities of materials in a chemical reaction● Molecular formula: Way to represent a molecule by containing the symbol of eachelement present and number of atoms present as a subscript.● Avogadro’s Number: One Mole○ Even a small amount of atoms/molecule on the macroscale contains a hugenumber of molecules○ Chemists often use a measuring unit called the mole○ Mole or Avogadro’s number is defined as■ 1 Mole of atoms = 6.022 x 10^23 atoms■ 602200000000000000000000 (huge number)● Molar mass: mass in grams of one mole of the element.○ One mole of element has mass in grams equal to its atomic weight.○ Atomic weight = molar mass● Atoms to moles = divide by 6.022 x 10^23 atoms● Moles to atoms = multiply by 6.022 x 10^23 atoms● Moles to mass(g) = multiply by atomic weight● Mass(g) to moles = divide by atomic weight● Molar Mass of Compounds/Molecules○ Molecular weight/mass of compound = take atomic weight of individual atomsand multiply by how many of those atoms there in the compound then add them.○ Mass in grams of 1 mole of the compound○ The molar mass of a molecule is the sum of the atomic weights of all the atoms inthe molecule○ Molar mass of compound is also referred as molecular weight or formula weightof the compound○ Example: What is the molar mass of NaCl?○ From the periodic table atomic weights of Na = 22.990 g/mol and Cl = 35.453g/mol○ Therefore the molar mass of NaCl = 58.443 g/mol○ One mole (6.022 x 1023 atoms) of NaCl weighs 58.443 g● Percent Composition by mass/weight○ Percentage composition is the percentage, by mass, of each element in thecompound○ Also referred to as percent by mass○ In other words, % composition of an element is the mass of an element presentin exactly 100g of compound○ % composition of an element = (Number of atoms of the element)(molar mass ofthe element)/molar mass of the compound x 100■ Ex: % weight of Carbon in C2H6 = (2) (12.01) / (30.08) x 100 = 79.85○ The sum of all the percent composition values for a compound must equal to 1● Empirical formula: Empirical formula is the simplest whole number ratio of elementspresent in the compound○ Unlike molecular formula, empirical formula does not provide information ofnumber of atoms in a molecule○ Ex: C6H12O6 (glucose) and C5H10O5 (ribose) have the same empirical formula(CH2O) but different molecular formula.● Empirical Formula from % Mass○ A pure compound always consists of the same elements combined in the sameproportions by weight○ We can determine the empirical formula from a compound’s elemental % mass○ Example: Hydrazine is composed of 87.42% Nitrogen and 12.58% Hydrogen.Molar mass = 32.06 g/mole. What is the empirical and molecular formula ofhydrazine?■ Step A: Determine the Empirical Formula■ Step B: Use empirical formula and molar mass to determine the molecularformula● Molecular Formula from Empirical Formula○ Step B: Use empirical formula and molar mass of compound (Mhydrazine =32.06 g/mol) to determine the molecular formula■ Step 1: Calculate the molar mass of the empirical formula.■ Step 2: Divide the molar mass that is given by the number obtained instep 1 and round your answer to the nearest whole number.■ Step 3: Multiply the subscript for each element in empirical formula by thewhole number obtained in step 2.● Chemical Equations○ (g) = gaseous (s) = solid (l) = liquid (aq) = aqueous which means dissolved inwater.○ Law of Conservation of Matter: Matter can be neither created nor destroyed.Atoms are conserved in chemical reaction.● Reaction Stoichiometry: The study of the relationships between the amount ofreactants and products on the macroscopic scale.● Limiting Reagent/Reactant: It is the reactant that runs out first in a chemical reaction○ When the limiting reactant is all used up, no more product can form, and thereaction stops■ To find limiting reactant:● Convert reactants into moles● Divide by Coefficient● Smallest number = limiting reactant.○ Excess reactant is what is left over after the reaction stops because the limitingreactant got all used up.● Percent Yield: The efficiency of a reaction is given by % yield.○ % Yield =( experimental yield / theoretical yield) x 100● Experimental yield: The actual amount of product produced in the experiment.Whatever is given.● Theoretical yield: The calculated amount of product from a given amount of

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