73533532 chemistry 311 lecture notes
TRANSCRIPT
-
8/12/2019 73533532 Chemistry 311 Lecture Notes
1/225
Chemistry 311
Lecture Notes
Fall 2009
David E. Henderson
-
8/12/2019 73533532 Chemistry 311 Lecture Notes
2/225
Chapter 2 & 3
How do we measure things
Chem 311
8/3/2009 Chem 311 Fall 2009 2
RECORDING DATA - USE OFSIGNIFICANT FIGURES
INTEGERS (Things you count) ALWAYS EXACT VALUES eg.you did the
experiment three times. Precision of these numbers is infinite.
REALS (Things you measure) ALWAYS CONTAIN AN UNCERTAINTY.
Assume error is +/- 1 in the last place unless specified. For the number 1.0034 the actual value lies between
1.0033 and 1.0035.
8/3/2009 Chem 311 Fall 2009 3
METHODS FOREXPRESSING
UNCERTAINTY ABSOLUTE UNCERTAINTY.
Buret reading 40.34 +/- .01 ml RELATIVE UNCERTAINTY.
0.01 ml/40.34 ml = .00025 relative .00025 * 100% = .025% rel.unc. .00025 * 1000 ppt= .25 ppt rel.unc. = 0.2 ppt
2-
-
8/12/2019 73533532 Chemistry 311 Lecture Notes
3/225
8/3/2009 Chem 311 Fall 2009 4
SIGNIFICANT FIGURES
KEEP ONLY THE FIRST UNCERTAINDIGIT IN A RESULT. Assume that data is presented this way
by other responsible scientists.
8/3/2009 Chem 311 Fall 2009 5
SIGNIFICANT FIGURES
ROUND OFF SUPERFLUOUSDIGITS. 5 ROUNDS UP IF ODD DOWN IF EVEN.
Thus the result of rounding the digit 5 is alwaysa number ending in an even digit. eg..435 =>.44 .425=> .42
Round off only once
8/3/2009 Chem 311 Fall 2009 6
SIGNIFICANT FIGURES
ADDING OR SUBTRACTING KEEP ONLY THE NUMBER OF DECIMAL
PLACES IN THE LEAST PRECISERESULT.(Remember these numbers willalways have the same units.)
Note- the number of significant digits in anumber can change dramatically insubtraction.
2-
-
8/12/2019 73533532 Chemistry 311 Lecture Notes
4/225
8/3/2009 Chem 311 Fall 2009 7
SIGNIFICANT FIGURES
MULTIPLYING OR DIVIDING Rule of Thumb Keep the same
number of significant digits in the resultas in the value with the lowest numberof sig. Figs.
This often doesnt work out 101/99 = 1.02
8/3/2009 Chem 311 Fall 2009 8
SIGNIFICANT FIGURES
COMMON ERROR 0.7834 gm KHP =0.0034 moles KHP
This error appears in far too many labnotebooks.The student has taken anumber with a relative uncertainty of1/7800 and reduced it to a number withan apparent uncertainty of 1/34.
8/3/2009 Chem 311 Fall 2009 9
Common error
1.234*(40.35-40.31)=0.04936 How many significant figures should we
have? 1.234*0.04= 0.04936 0.05
2-
-
8/12/2019 73533532 Chemistry 311 Lecture Notes
5/225
8/3/2009 Chem 311 Fall 2009 10
SIGNIFICANT FIGURES
DURING CALCULATIONS ONE ADDITIONAL INSIGNIFICANT DIGITSHOULD BE CARRIED ININTERMEDIATE RESULTS.
EXCEPTION.The calculation of sumsand sums of squares requires you tokeep all digits
8/3/2009 Chem 311 Fall 2009 11
LOGS
log consists of two parts, characteristic
eg.pH=1.03 characteristic = 1 mantissa = .03 The characteristic is a power of ten and is an
integer (counting decimal pl aces) .
mantissa. Measured value - real The significant figures which denote the
uncertainty of the value are those in themantissa.
8/3/2009 Chem 311 Fall 2009 12
LOGS - General rule Number of Sig. Figs is number of digits to
right of decimal A two digit mantissa corresponds to a relative
error of approximately 2%,
A three digit mantissa to 2 ppt, etc. EXAMPLE
pH 1.03 [H +]=9.3x10 -2 pH 13.03 [H +] = 9.3x10 -14
1.02 < pH < 1.04 13.02 < pH < 13.04 9.1x10 -2 < [H +] < 9.5x10 -2 9.1x10 -14 < [H +]