Chem. 31 – 8/30 Lecture

Announcements I Adding Lecture Only Status Sections are more full than I expected Sections 4, 5, 6, and 7 have had some drops (so top of waitlist will be added) Sometimes 1 more student per section will drop Lecture Only Status Students repeating the class that received an 80% or better on the lab can qualify for “lecture only” (can use previous lab score – see me if you think you qualify

Announcements II Homework and Quiz quiz next Wednesday (next class meeting – just on material covered today) corrected diagnostic quiz also due Wed. (can put answers on the back) Must attend a safety lecture (if absent from your lab section, attend another safety lecture) Lab Procedure’s Quiz Sect. 5 to 7, Sept. 7th Sect. 2 to 4, Sept. 11th

Traditional vs. Modern Methods Characteristic Traditional Modern Equipment Glassware and balances (low cost) Instruments (high cost) Precision High Moderate Speed slow fast Sensitivity low high Selectivity minimal Good 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) Measure Unit_____ Length meter (m) Mass kilogram (kg) (only one with multiplier) Time second (s) Temperature Kelvin (K) Amount Mole (mol)

Units of Measure Directly Derived from Base Units Volume: cube volume = l3 so units = m3 Density = m/V so kg/m3 Pressure = force(kg·m/s2)/area(m2) = kg/(s2·m) l

Units of Measure Other metric units (not directly in SI units) Density (g/cm3) Pressure (Pascals or Pa = kg/(s2·m)) Non-metric units (used commonly) For pressure 1 atmosphere (atm) = 101300 Pa English/Other system (not emphasized here)

Units of Measure Metric Multipliers (ones you should know) Name Abbreviation Multiplier Kilo k x103 Centi c x10-2 Milli m x10-3 Micro m x10-6 Nano n x10-9 Analytical chemists like small quantities. An instrument that can detect 1 fg (1 x 10-15 g) is better than an instrument that can detect 1 pg (1 x 10-12 g)

Unit Conversion – Example Problem Convert the density of lead from g/cm3 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 H2O2 solution that is listed as 25% H2O2 by mass and has a density of 1.07 g mL-1? MW (H2O2) = 34.0 g mol-1

Preparing Solutions From Scratch (direct from solids) Let’s say we want 100 mL of 0.100 M FeCl3. What equipment do we need? How do we make the solution?

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

Preparing Solutions By Dilution What if we need 20.0 mL of 0.00200 M FeCl3? How do we make it? If direct by weighing, mass FeCl3 = 0.0065 g (not very accurate) By Dilution from 0.100 M FeCl3

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