Unit 1: Introduction to

Chemistry

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Types of Measurements and Observations, Scientific Notation

Day 3 - Notes

• Measurement: a type of observation

• Qualitative measurements: descriptive–Ex: hot, cold, heavy, light, big,

blue, furry• Quantitative measurement:

observation made with a measuring instrument and includes both a number and a unit–Ex: ruler, balance, thermometer,

graduated cylinder, 13.5°C, 25kg, 17L

• Accuracy: How close a measurement is to the true or accepted value–Ex: Weighing a 50g mass

50.00g – accurate32.18g – not accurate49.99g – accurate

• Precision: How close multiple measurements are to each other– Ex: Take the weight of a 50g mass

Accurate, precise: Accurate, precise:50.00g 50.00g50.00g 49.99g50.00g 50.00g

Not accurate, precise:32.18g32.18g32.18g

An easy way to remember…

ACcurate = CorrectPRecision =

Reproducibility

Significance in Measurement

. Which of the following best describes the length of the beetle's body in the picture to the left?

a) Between 0 and 2 in b) Between 1 and 2 inc) Between 1.5 and 1.6 ind) Between 1.54 and 1.56

ine) Between 1.546 and

1.547 in

Significance in Measurement

The correct answer is . . .

(d), between 1.54 and 1.56 inches

Significance in Measurement

Measurements are often written as a single number rather than a range. The beetle's length in the previous frame was between 1.54 and

1.56 inches long. The single number that best represents the measurement is the

center of the range, 1.55 inches. When you write the measurement as a single number, it's

understood that the last figure (the second of the two 5’s in this case) had to be estimated. Consider measuring the length of the same object with two different rulers.

Significance in Measurement

. For each of the rulers, give the correct length measurement for the steel pellet as a single number rather than a range

Significance in Measurement

For the ruler on the left you should have had . . .1.4 in

For the ruler on the right, you should have had . . .1.47 in

Significance in Measurement

A zero will occur in the last place of a measurement if the measured value fell exactly on a scale division. For example, the

temperature on the thermometer should be recorded as 30.0°C.

Reporting the temperature as 30°C would imply that the measurement had been taken on a thermometer with scale marks 100°C apart!

Significance in Measurement Use the bottom of the

meniscus (the curved interface between air and liquid) as a point of reference in making measurements of volume in a graduated cylinder, pipet, or buret.

In reading any scale, your line of sight should be perpendicular to the scale to avoid 'parallax' reading errors.

Significance in Measurement

The graduated cylinder on the right has scale marks 0.1 mL apart, so it can be read to the nearest 0.01 mL.

Reading across the bottom of the meniscus, a reading of 5.72 mL is reasonable (5.73 mL or 5.71 mL are acceptable, too).

Significance in Measurement

Determine the volume readings for the two cylinders to the right, assuming each scale is in mL.

Significance in Measurement

For the cylinder on the left, you should have measured . . .

3.0 mL For the cylinder on the right, you should have

measured . . .0.35 mL