Unit 2: Quantities in Chemistry


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1 Mass, Moles, & Molar Mass Relative quantities of isotopes in a natural occurring element (%) E.g. Carbon has 2 isotopes C12 and C13. Of Carbon s two isotopes, there is 98.9% C12 and 11.1% C13. Find the average atomic mass of Carbon. Unit 2: Quantities in Chemistry Section: Pages Formula mass unit E.g. Calculate the formula mass of CaCl
2 The Mole Periodic Table and Mass of Elements Mole Molar Mass, M The number of entities in one mole, x E.g. Calculate the molar mass of NaCl. How many jellybeans are in a box that contains 250 g of jellybeans, if 15 jellybeans have a mass of 10.0g? Step 1: Identify Key Value and Conversion Factor for Equation. Step 2: Identify Required Value. Step 3: List Possible Conversion Factors. Step 4: Substitute Values into Equation and Solve. 3 4
3 Number & Amount of Entities Calculations Involving the Mole n  m m n M M  E.g., Calculate the mass, in grams, of 2.00 moles of calcium atoms. How many oxygen molecules, O 2(g), are in a cylinder that contains 48.0 g of oxygen if x molecules of oxygen have a mass of g? Step 1: Identify Key Value & Conversion Factor Equation. Step 2: Identify Required Value. E.g. What amount of gold is in a 275.8g nugget of pure gold? And how many atoms does this represent? Step 3: List Possible Conversion Factors. Step 4: Substitute Values into Solution Equation and Solve. 5 6
4 Calculations Involving the Mole Continued. Calculate the mass of sodium hydrogen carbonate (baking soda), NaHCO 3. Sodium fluoride is added to toothpaste and tap water to prevent tooth decay. Calculate the mass of 2.00 mol of sodium fluoride, NaF (s) Determining Empirical Formulas Percentage Composition A compound contains elements in certain fixed proportions. E.g. NaCl, H 2 O, C 6 H 12 O 6 A formula that gives the lowest ratio of the atoms in a compound The percentage composition of a compound was found to be 69.9% iron and 30.1% oxygen. What is the empirical formula of the compound. Step 1: List given values. Step 2: Calculate Mass (m) of each element in a 100 g sample. Step 3: Convert Mass (m) into Amount in moles (n) How many water molecules are in a 25.0g sample of water, H 2 O (l)? Step 4: State Amount ratio. Step 5: Calculate Lowest WholeNumber Ratio. 7 8
5 Determining Molecular Formulas Determining Percent Composition A formula that indicates the actual number of atoms in one molecule of a compound. Calculate the percentage composition of potassium sulfate, K 2 SO 4. Step 1: Calculate the Total Mass of Each Element in the Compound. The empirical formula of a compound is CH 3 O, and its molar mass is g/mol (determined by a mass spectrometer). What is the molecular formula of the compound? Step 1: List given values. Step 2: Calculate Molecular Mass (or formula unit mass) of Compound. Step 2: Determine Molar Mass of the Empirical Formula. Step 3: Determine Molar Mass of Compound to Molar Mass of Empirical Formula. Step 3: Calculate Percentage Composition by Mass of Compound. Step 4: Calculate Molecular Formula. 9 10
6 Quantitative analysis % Concentration V/V, W/V A ratio of the quantity of solute in a solution. Molar Concentration c is the molar concentration in mol/l. n is the amount of solute in moles. V is the volume of the solution in L. A sodium hydroxide solution contains mol of sodium hydroxide in L of solution. Calculate the molar concentration of the sodium hydroxide solution. Step 1: List Given Values. Step 2: Write Molar Concentration Equation, Substitute Values, & Solve. A salt solution is formed by mixing 2.80 g of NaCl (s), in enough water to make exactly 250 ml of solution. What is the W/V percentage concentration of sodium chloride salt solution? Step 1: List Given Values. Step 2: Write Percentage Concentration Equation, Substitute Values, & Solve
7 Parts Per Million Concentration unit that is used for very low concentrations; one part solute for every million parts of solvent. Concentrations of Solutions Summary! In a chemical analysis 2.2 mg of oxygen was measured in 250 ml of pond water. What is the concentration of oxygen in ppm? Step 1: List Given Values. Step 2: Write Percentage Composition Equation. Step 3: Make Necessary Metric Conversion. Step 4: Substitute Values into Equation and Solve
8 Diluting Aqueous Solutions Dilution Molar Ratio Stoichiometry Mathematical procedures for calculating the quantities of reactants and products involved in chemical reactions. Propane, C 3 H 8(g), is a gas that is commonly used in barbecues. Calculate the mass of oxygen that is required to burn 15.0 g of propane. Step 1: Write the Unbalanced Chemical Equation. Step 2: Balance Equation, List Given Values and Molar Masses. c i initial concentration V i initial volume c i v i = c f v f c f final concentration V f final volume Calculate the final volume of a hydrogen peroxide solution if water is added to a 100 ml of 6% V/V hydrogen peroxide solution until it reaches a volume of 250 ml. Step 1: List Given Values. Step 2: Write Dilution Equation. Step 3: Convert Mass of a Given Substance to an Amount (moles). Step 4: Convert Amount of a Given Substance to Amount Required of a Given Substance. Step 3: Isolate Unknown Value on LeftHand Side. Step 4: Substitute Values in & Solve. Step 5: Convert Amount of Required Substance to Required Value
9 Summary of Stoichiometry! Limiting & Excess Reagents Limiting Reagent The reactant that is present in more than the required amount in a chemical reaction. Table salt, NaCl (s), can be formed by the reaction of sodium metal with chlorine gas: 2Na (s) + Cl 2(g) 2NaCl (s) A reaction mixture contains g of sodium and g of chlorine. Calculate the mass of sodium chloride that is produced. Step 1: List Given Values. Step 2: Complete Reaction Chart. Step 3: Identify Limiting & Excess Reagents. Step 4: Calculate Amount of Limiting Reagent. Step 5: Calculate the Amount of Product. 17 Step 6: Calculate the Mass of Product. 18
10 Percentage Yield Actual Yield The amount of product produced in a chemical reaction. Percentage Yield The amount of product expected from a balanced chemical equation. The most common ore of Arsenic is FeSAs (s), can be heated to produce Arsenic, As (s) : FeSAs (s) FeS (s) + As (s) When 250 kg of this ore was processed industrially, 95.3 kg of Arsenic was obtained. Calculate the percent yield of Arsenic. Step 1: Write Unbalanced Equation. Step 2: Balance Equation, List Values & Molar Masses. Step 3: Convert Mass of Given Substance to Amount of Substance (n) Step 4: Convert Amount of Substance to Amount of Required Sub. Step 5: Convert Amount of Required Sub to Mass of Req. Sub. Step 6: Calculate Percent Yield. 19
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