SIGNIFICANT figures
Two types of numbers: exact and inexact. Exact numbers are obtained by counting or by definitions – a dozen of wine, hundred cents in a dollar
All measured numbers are inexact.
Learning objectives
Define accuracy and precision and distinguish between them
Make measurements to correct precision Determine number of SIGNIFICANT FIGURES in a
number Report results of arithmetic operations to correct
number of significant figures Round numbers to correct number of significant
figures
All analog measurements involve a scale and a pointer
Errors arise from:– Quality of scale– Quality of pointer– Calibration– Ability of reader
ACCURACY and PRECISION
ACCURACY: how closely a number agrees with the correct value
PRECISION: how closely individual measurements agree with one another – repeatability
– Can a number have high precision and low accuracy?
Significant figures are the number of figures believed to be correct
In reading the number the last digit quoted is a best estimate. Conventionally, the last figure is estimated to a tenth of the smallest division
2.0 2.1 2.2 2.3 2.4 2.5
2.3 6
The last figure written is always an estimate
In this example we recorded the measurement to be 2.36
The last figure “6” is our best estimate It is really saying 2.36 ± .01
2.0 2.1 2.2 2.3 2.4 2.5
Precision of measurement (No. of Significant figures) depends on scale – last digit always estimated
Smallest Division = 1 Estimate to 0.1 – tenth of smallest division 3 S.F.99.6
97 98 99 100
Lower precision scale
Smallest Division = 10 Estimate to 1 – tenth of smallest division 2 S.F.96
70 80 90 100
Precision in measurement follows the scale
Smallest Division = 100 Estimate to 10 – tenth of smallest division 1 S.F.90
0 100
Measuring length
What is value of large division?
– Ans: 1 cm What is value of small
division?– Ans: 1 mm
To what decimal place is measurement estimated?
– Ans: 0.1 mm (3.48 cm)
Scale dictates precision
What is length in top figure?
– Ans: 4.6 cm
What is length in middle figure?
– Ans: 4.56 cm
What is length in lower figure?
– Ans: 3.0 cm
Measurement of liquid volumes
The same rules apply for determining precision of measurement
When division is not a single unit (e.g. 0.2 mL) then situation is a little more complex. Estimate to nearest .02 mL – 9.36 ± .02 mL
Reading the volume in a burette
The scale increases downwards, in contrast to graduated cylinder
What is large division?– Ans: 1 mL
What is small division?– Ans: 0.1 mL
RULES OF SIGNIFICANT FIGURES
Nonzero digits are always significant 38.57 (four) 283 (three)
Zeroes are sometimes significant and sometimes not
– Zeroes at the beginning: never significant 0.052 (two)– Zeroes between: always 6.08 (three)– Zeroes at the end after decimal: always 39.0 (three)– Zeroes at the end with no decimal point may or may
not: 23 400 km (three, four, five)
Scientific notation eliminates uncertainty
2.3400 x 104 (five S.F.) 2.340 x 104 (four S.F.) 2.34 x 104 (three S.F.)
23 400. also indicates five S.F. 23 400.0 has six S.F.
Note: significant figures and decimal places are not the same thing
38.57 has four significant figures but two decimal places
283 has three significant figures but no decimal places
0.0012 has two significant figures but four decimal places
A balance always weighs to a fixed number of decimal places. Always record all of them
– As the weight increases, the number of significant figures in the measurement will increase, but the number of decimal places is constant
– 0.0123 g has 3 S.F.; 10.0123 g has 6 S.F.
Significant figure rules
Rule for addition/subtraction: The last digit retained in the sum or difference is determined by the position of the first doubtful digit
37.24 + 10.3 = 47.51002 + 0.23675 = 1002225.618 + 0.23 = 225.85
Position is key
Significant figure rules
Rule for multiplication/division: The product contains the same number of figures as the number containing the least sig figs used to obtain it.
12.34 x 1.23 = 15.1782
= 15.2 to 3 S.F.
0.123/12.34 = 0.0099675850891
= 0.00997 to 3 S.F. Number of S.F. is key
Rounding up or down?
5 or above goes up– 37.45 → 37.5 (3 S.F.)– 123.7089 → 123.71(5 S.F.); 124 (3 S.F.)
< 5 goes down– 37.45 → 37 (2 S.F.)– 123.7089 → 123.7 (4 S.F.)
Scientific notation simplifies large and small numbers
1,000,000 = 1 x 106
0.000 001 = 1 x 10-6
234,000 = 2.34 x 105
0.00234 = 2.34 x 10-3
Multiplying and dividing numbers in scientific notation
(A x 10n)x(B x 10m) = (A x B) x 10n + m
(A x 10n)/(B x 10m) = (A/B) x 10n - m
Adding and subtracting
(A x 10n) + (B x 10n) = (A + B) x 10n
(A x 10n) - (B x 10n) = (A - B) x 10n