Chem. 31 – 8/31 Lecture

Announcements No class Monday (Labor Day), but lab on Tuesday (Tu/Th labs will be “ahead” of M/W labs until after Thanksgiving) Homework and Quiz –quiz next Wednesday –corrected diagnostic quiz also due Wed (can put answers on the back) –additional problem 1.1 also due

Traditional vs. Modern Methods CharacteristicTraditionalModern EquipmentGlassware and balances (low cost) Instruments (high cost) PrecisionHighModerate Speedslowfast Sensitivitylowhigh SelectivityminimalGood to great

Chapter 1 – Measurements and Titrations No measurement is valuable unless it is given with units and some measure of uncertainty Units – Chapter 1 Uncertainty – Chapters 3 and 4

Units of Measure Most Basic – SI base units (important ones) MeasureUnit_____ Lengthmeter (m) Masskilogram (kg) (only one with multiplier) Timesecond (s) TemperatureKelvin (K) AmountMole (mol)

Units of Measure Directly Derived from Base Units –Volume: cube volume = l 3 so units = m 3 –Density = m/V so kg/m 3 –Pressure = force(kg·m/s 2 )/area(m 2 ) = kg/(s 2 ·m) l

Units of Measure Other metric units (not directly in SI units) –Density (g/cm 3 ) –Pressure (Pascals or Pa = kg/(s 2 ·m)) –Volume (mL) Non-metric units (used commonly) –For pressure 1 atmosphere (atm) = Pa –English/Other system (not emphasized here)

Units of Measure Metric Multipliers (ones you should know) NameAbbreviationMultiplier Kilokx10 3 Centicx10 -2 Millim x10 -3 Micro  x10 -6 Nanonx10 -9 Analytical chemists like small quantities. An instrument that can detect 1 fg (1 x g) is better than an instrument that can detect 1 pg (1 x g)

Unit Conversion – Example Problem Convert the density of lead from g/cm 3 to kg m -3 if density = 11.7 g cm -3.

Concentration Units General form Note: sometimes, volume is required in the denominator to be strictly considered “concentration”, but for this class mass ratios or mole ratios will be considered to be an expression of concentration

Most Commonly Used Concentration Units Molarity (M) Mass Fraction (also valid for solids as mass analyte/mass sample)

Most Commonly Used Concentration Units Mass Fraction – continued Other Units: Mass/volume units (e.g. mg/mL)

Conversion between Concentration Units – Example Problem What is the molarity of a H 2 O 2 solution that is listed as 25% H 2 O 2 by mass and has a density of 1.07 g mL -1 ? MW (H 2 O 2 ) = 34.0 g mol -1

Preparing Solutions From Scratch (direct from solids) Let’s say we want 100 mL of M FeCl 3. What equipment do we need? How do we make the solution?

Preparing Solutions From scratch 1. Solid to flask 3. Fill to line 2. Add liquid, dissolve

Preparing Solutions By Dilution What if we need 20.0 mL of M FeCl 3 ? How do we make it? If direct by weighing, mass FeCl 3 = g (not very accurate) By Dilution from M FeCl 3

Preparing Solutions By Dilution 1. Pipet liquid 2. Add liquid, fill to mark

Stoichiometry Stoichiometry refers to ratios between moles of reactants and products in chemical reactions The ratio of moles of reactants and products is equal to the ratio of their stoichiometric coefficients Example:aA + bB ↔ cC + dD Moles A/moles B = a/b

Stoichiometry Example problem: How many moles of H 2 O 2 are needed to completely react with 25 mL of 0.80 M MnO 4 - ? Reaction: 5H 2 O 2 (aq) + 2MnO H + ↔ 2Mn O 2 (g) + 8H 2 O(l)

Stoichiometry Remember: there are two (common) ways to deliver a known amount (moles) of a reagent: –Mass (using formula weight) –Volume (if molarity is known)