Scientific Measurement

• What is density? • From your

experimental data, were the densities of the similar objects the same or different? Why?

• What does this tell

you about density?

• Can you look up the

density of a particular substance?

• Does the size of the

substance play a role in changing its density?

• For the irregular

shaped objects, did you get similar densities for each? Why or why not? If you didn’t, can you give a reason as to why? (accuracy and measuring tools)

Scientific Measurement

• Qualitative and Quantitative • What is the difference between qualitative

and quantitative measurements?

• Qualitative- results that are descriptive and

nonnumeric

• Quantitative- results are given in a definite

form, usually as numbers and units

• Most of the things that we will be doing in

chemistry will be quantitative, but there will qualitative elements as well

Scientific Notation (Review)

• What is scientific

notation?

• Example:

11000000000m

• s.n. : 1.1 *10

10 m

• Example: 8.1 *10

-3 m = 0.0081m

• Example: diameter

of a hair: 0.000008m = 8.0*10 -6 m

• Multiplication • 3.0 *10

6 x 2.0*10 *2.0) x 10 (6+3) 3 = (3.0 = 6.0*10 9

• 2.0 *10

*10 (-3+5) -3 x 4.0 *10 = 8.0*10 2 5 = 8.0

• (Add exponents) • Division • 3.0*10

4 /2.0*10 2 3.0/2.0 x10 (4-2) *10 2 = = 1.5

• 6.0*10

-2 /2.0*10 3.0*10 (-2-4) 4 = = 3.0*10 -6

• (Subtract denominator

from the numerator)

Accuracy and Precision, Percent Error

• Accuracy- measure

of how close a measurement comes to the actual or true value of whatever is being measured

• Precision- measure

of how close a series of measurements is to one another

• Percent error

compares the experimental value to the correct value

• Accepted value-

correct value based on reliable references, what types of references, your neighbor?

• Experimental value-

value measured in the lab

Accuracy and Precision

Percent Error

• Difference between

accepted and experimental values is called error

• Error= accepted

value-experimental value

• % Error=

[error]/accepted value * 100%

• Density of water=

1.0 g/mL (accepted)

• 0.98 g/mL

(experimental)

• Percent Error =

[1.0 g/mL 0.98g/mL]/1.0 g/mL * 100% = 2%

Significant Figures in Measurements

• The calibration of your measuring tool

determines how many sig. Figs. you can have.

Significant Figures in Measurements

•

Example #1:

• • • •

This ruler measures to the .1 (in this case centimeters) However, I can see that the measurement lies between the 2.8 and 2.9 measurement, so I can make the estimate that it is approximately 2.83 cm. You see!!! All of those numbers are significant, because they all tell me about the measurement!

If I went out any further, it would not be accurate, because my measuring device is not that accurate!

Significant Figures in Measurements

• This works for other measuring

devices as well. Just remember to always go one digit further than the device does

• Example #1: • What temp does the

thermometer on the left indicate?

• The thermometer has whole

number digits , so for sig figs I can go to the tenths.

• The temp is 28.5

o C

Significant Figures in Measurements

• This also works for Graduated Cylinders • Example #1 • The drawing above indicates you are

looking at a graduated cylinder from the side (note the dip or meniscus, which you always read from the bottom)

• This graduated cylinder measures to the

whole number so we will read it to the tenth

• This graduated cylinder has a reading of

30.0 ml

Rules for Significant Figures

• • • • Every nonzero digit reported in •

measurement is assumed to be significant

• How many sig. Figs.?

-24.7m

• • •

-0.743m

-714m

• three • Zeros appearing between nonzero digits

are significant

• How many sig. Figs.?

-7003m -40.79m

-1.503m

Four

Rules for Significant Figures

• Leftmost zeros appearing in front of nonzero •

digits are not significant (Act as placeholders)

• 0.0071m • 0.42m • 0.000099m

two

• Zeros at the end of a number and to the right

of a decimal point are always significant.

• 43.00m • 1.010m • 9.000m •

Four

Rules for Significant Figures

• Zeros at the rightmost end of a

measurement that lie to the left of an understood decimal point are not significant if they serve as placeholders to show the magnitude of the number.

• 300m (1) • 7000m (1) • 27210m (4) • If 300 was found from careful measurement

and not a rough guess, then the zeros would be significant. To avoid this, write in scientific notation.

• 3x10

2 m - not significant

• 3.00x10

2 m – significant

Significant Figures in Calculations

• Calculated values cannot be more

precise than the measured values used to obtain it.

• Addition and Subtraction • round to the same number after the

decimal place as the measurement with the least number after the decimal place.

• 12.54m + 349.0m + 8.24m = 369.76m =

369.8m

• 74.626m – 28.34m = 46.286m = 46.29m

Significant Figures in Calculations

• Multiplication and Division • round answer to the same # of

significant figures as the measurement with the least # of significant figures.

• 7.55m * 0.34m = 2.6 (2) • 0.365m * 0.0200m = 0.00730 (3) • 2.4526m / 8.4m = 0.29 (2)

SI Units

• Factor Name Symbol • 10

-1 deci d

• 10

-2

• 10

-3

• 10

-6

• 10

-9

• 10

-12

• 10

-15

• 10

-18 centi c milli m micro µ nano n pico p femto f atto a

SI Units

• Factor Name • 10

6 Symbol mega M

• 10

3

• 10

2

• 10

1 kilo k hecto h deka da

Glassware • Which are used to measure

approximate volumes?

• Which are used to measure more

precise volumes?

• Which one would you use to

measure a large volume, such as 100 mL, accurately?