Chem. 31 – 9/7 Lecture

Announcements I Today –Quiz (after announcements) –Turn in corrected diagnostic quiz (only if you got less than a 12 and want more points) –Turn in Additional Problem 1.1 –Put your lab section on all assignments! In Lab Thursday and next Monday – Lab Procedures Quiz (covers lab lectures, introductory parts of lab manual, safety lectures, and lab grading) Adding (No vacancies – check this again)

Announcements II SacCT and Website –Website will contain most of the information needed for the class –I intend to use SacCT Sect 1 (lecture) to post keys for quizzes, homework and exams (basically to reduce this from public posting) –I intend to use SacCT lab sections for posting grades (this is done in Chem 1B, but I’m trying this in Chem 31 for the first time)

Announcements III Today’s Lecture –Stoichiometry (end of Chapter 1) –Error and Uncertainty (Chapter 3) Definitions Significant figures Accuracy and precision in measurements

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)

Chapter 3 – Error and Uncertainty Error is the difference between measured value and true value or error = measured value – true value Uncertainty –Less precise definition –The range of possible values that, within some probability, includes the true value

Measures of Uncertainty Explicit Uncertainty: Measurement of CO 2 in the air: ppmv The + 3 ppm comes from statistics associated with making multiple measurements (Covered in Chapter 4) Implicit Uncertainty: Use of significant figures (399 has a different meaning than 400 and )

Significant Figures (review of general chem.) Two important quantities to know: –Number of significant figures –Place of last significant figure Example: significant figures and last place is hundredths Learn significant figures rules regarding zeros

Significant Figures - Review Some Examples (give # of digits and place of last significant digit) –21.0 –0.030 –320 –10.010

Significant Figures in Mathematical Operations Addition and Subtraction: –Place of last significant digit is important (NOT number of significant figures) –Place of sum or difference is given by least well known place in numbers being added or subtracted Example: Hundredths placeones place = Least well known = 15

Significant Figures in Mathematical Operations Multiplication and Division –Number of sig figs is important –Number of sig figs in Product/quotient is given by the smallest # of sig figs in numbers being multiplied or divided Example: 3.2 x places5 places = = 520 = 5.2 x 10 2

Significant Figures in Mathematical Operations Multi-step Calculations –Follow rules for each step –Keep track of # of and place of last significant digits, but retain more sig figs than needed until final step Example: (27.31 – 22.4)2.51 = ? Step 1 (subtraction): (4.91)2.51 Step 2 multiplication = = 12 Note: 4.91 only has 2 sig figs, more digits listed (and used in next step)

Significant Figures More Rules Separate rules for logarithms and powers (Covering, know for homework, but not tests) –logarithms: # sig figs in result to the right of decimal point = # sig figs in operand example: log(107) –Powers: # sig figs in results = # sig figs in operand to the right of decimal point example: = operand 3 sig fig = results need 3 sig figs past decimal point = = 2.51 x = 3 x sig fig past decimal point

Significant Figures More Rules When we cover explicit uncertainty, we get new rules that will supersede rules just covered!

Types of Errors Systematic Errors –Always off in one direction –Examples: using a “ stretched ” plastic ruler to make length measurements (true length is always greater than measured length); reading buret without moving eye to correct height Random Errors –Equally likely in any direction –Present in any (continuously varying type) measurement –Examples: 1) fluctuation in readings of a balance with window open, 2) errors in interpolating (reading between markings) buret readings True Volume Meas. Volume eye

Accuracy and Precision Accuracy is a measure of how close a measured value is to a true value Precision is a measure of the variability of measured values Precise and Accurate Precise, but not accurate Poor precision (Accuracy also not great)

Accuracy and Precision Accuracy is affected by systematic and random errors Precision is affected mainly by random errors Precision is easier to measure

Accuracy and Precision Both imprecise and inaccurate measurements can be improved Accounting for errors improves inaccurate measurements (if shot is above and right aim low + left) Averaging improves imprecise measurements aim here rough ave of imprecise shots