Scientific Measurement - Central Valley School District

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Transcript Scientific Measurement - Central Valley School District

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 QuantitativeWhat 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

Multiplication3.0 *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)Division3.0*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 CylindersExample #1The 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

threeZeros 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)

0.0071m0.42m0.000099m

two

Zeros at the end of a number and to the right

of a decimal point are always significant.

43.00m1.010m9.000m

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 Subtractionround 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 Divisionround 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 Name10

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?