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Wednesday, Oct. 2 nd : “A” Day Thursday, Oct. 3 rd : “B” Day Agenda  Homework questions/collect (pg. 53: #1-7)  Sec. 2.2 quiz: “Studying Matter and Energy”  Begin Section 2.3: “Measurements and Calculations in Chemistry” Anticipation Guide Accuracy, Precision, Significant Figures  Homework: “Significant Figures Practice” Worksheet “Additional Problems” Worksheet

Homework  Sec. 2.2 review, pg. 53: #1-7  Questions/problems?

Section 2.2 Quiz “Studying Matter and Energy”  You may use your guided notes and your book to take this quiz with a partner of your choice…. May the force be with you!

Sec. 2.3 Anticipation Guide  Complete the anticipation guide to the best of your ability before we begin section 2.3.  Any you get correct will by X-credit!

Accuracy and Precision  No value that is obtained from an experiment is exact because all measurements are subject to limits and errors.  Human errors, method errors, and the limits of the instrument are a few examples.  To reduce the impact of error on their work, scientists always repeat their measurements and calculations a number of times.

Measurements Must Involve the Right Equipment  Selecting the right piece of equipment to make your measurement is the first step to cutting down on errors in experimental results.

Accuracy is How Close a Measurement is to the True Value  Accuracy: a description of how close a measurement is to the true value of the quantity measured.  “How close am I to the bulls eye?”

Precision is How Closely Several Measurements Agree  Precision: the exactness of a measurement  How close together are several measurements of the same quantity that are measured the same way?

Accuracy and Precision High Accuracy Low AccuracyLow Accuracy High Precision High PrecisionLow Precision

Significant Figures (Sig Figs)  Significant Figure: a prescribed decimal place that determines the amount of rounding off to be done based on the precision of the measurement.  Sig figs consist of all digits known with certainty as well as one estimated digit.  The last digit or sig fig reported after a measurement is estimated.  In the penny lab, the volume of water was recorded to the tenths place, which was an estimated digit since the graduated cylinder only had markings every 1 mL.

Rules for Determining Sig Figs 1.Non-zero digits are ALWAYS significant. 2.Zeros between non-zero digits are significant. 3.Zeros in front of non-zero digits are NOT significant. (They are just placeholders) 4.Zeros both at the end of a number and to the right of a decimal point ARE significant. 5.Zeros both at the end of a number but to the left of a decimal point may not be significant. If a zero has not been measured or estimated, it is not significant. A decimal placed after zeros indicates that the zeros are significant.

Sig Fig Examples HHow many sig figs are in the following examples? grams 4 sig figs liters 5 sig figs meters 3 sig figs 5.00 kg 3 sig figs

More Sig Fig Examples  How many sig figs are in the following examples? 218 kPa 3 sig figs L 2 sig figs 200. m 2 3 sig figs 1.05 g 3 sig figs

How Do I Use Sig Figs to Round?  Round each to 3 significant figures: km 32.1 km g 156 g cm cm (the zero before the 2 is not significant)

Rules for Using Sig Figs in Calculations  In multiplication and division problems, the answer can’t have more sig figs than there are in the measurement with the least # of sig figs.  In addition and subtraction problems, the answer can’t have more digits to the right of the decimal point than there are in the measurement with the least # of digits to the right of the decimal.  If a calculation has both addition/subtraction and multiplication/division, round after each operation.

Calculators DO NOT Identify Sig Figs  Calculate the density of isopropyl alcohol, which has a volume of 32.4 mL and a mass of g.  Does your calculator show ?  How many sig figs does the mass have? 4  How many sig figs does the volume have? 3  Since this is a division problem, the answer can’t have more sig figs than there are in the measurement with the least # of sig figs.  The answer must be rounded to 3 sig figs, or g/mL.

Sig Figs in a Nutshell  Basically, your answer CAN’T have more sig figs than your LEAST precise measurement has!

Sig Fig Example #1  Calculate the following and show the answer with the correct number of sig figs: mL – mL = mL This is a subtraction problem, so the answer should be rounded to have the same # of digits to the right of the decimal place as the value with the least # of digits to the right of the decimal. This should be rounded to mL (4 digits to right of decimal point)

Sig Fig Example #2  Calculate the following and show the answers with the correct number of sig figs: (12.4 cm X cm) cm 2 (Perform each operation separately/round after each) 12.4 cm X cm = cm 2 Round to 3 sig figs: 98.5 cm cm cm 2 = cm 2 Round to 1 digit after decimal: 98.5 cm 2

Example Problem A, Pg. 59  A student heats g of a solid and observes that its temperature increases from 21.6˚C to 36.79˚C. Calculate the temperature increase per gram of solid. (Hint: round after each operation) Temperature increase = 36.79˚C – 21.6˚C = 15.19˚C This is rounded to 15.2˚C (1 digit after decimal) Temp change/gram of solid= 15.2˚C g = ˚C/g This is rounded to ˚C/g (3 sig figs)

Exact Values Have Unlimited Sig Figs  Some values that you will use in your calculations have no uncertainty.  These values have an unlimited number of sig figs and are exact values.  Examples of exact values are: Count values Conversion factors

Homework  “Significant Figures Practice” worksheet  “Additional Problems” worksheet  Don’t forget about your serendipitous discoveries paper…..  You MAY work with a partner if you’d like… Next Time: More Sig Fig Fun!