Homework: Due Friday 9/4

Do 1-10 on a separate sheet of paper, include units.

Do all problems on slide 25 as well.

SI Units

Quantity Base Unit Symbol

Length

Mass

Time

Temp

meter

kilogram

second

kelvin

m

kg

s

K

Amount mole mol

Accuracy vs. Precision• Accuracy - how close a measurement is

to the true value

• Precision - how close a set of measurements are to each other

ACCURACY = CORRECTNESS

PRECISION = CONSISTENCY

Percent Error

• Indicates accuracy of a measurement.

100literature

literaturealexperimenterror %

Percent Error• Julie determines the density of copper to

be 8.25 g/cm3. Find the % error if the actual density of copper is 8.92 g/cm3.

100g/cm 8.92

g/cm 8.92g/cm 8.25error %

3

33

% error = 7.51 %

Precision

• This is the repeatability of a measurement. • It may or may not be accurate • But everyone who does it gets the same

answer

Accuracy vs. Precision

Which of the following will allow measurement of a liquid’s volume with the greatest precision?

A 50 mL cylinder graduated in 1 mL increments

B 50 mL cylinder graduated in 0.5 mL increments

C 100 mL cylinder graduated in 1 mL increments

D 200 mL cylinder graduated in 5 mL increments

Answer: B

Significant Figures (Sig Figs)• Indicate precision of a measurement.

• Recording Sig Figs– Sig figs in a measurement include the known

digits plus a final estimated digit

2.33 cm

• If the decimal is present, start on the Pacific side at the first nonzero digit and count it and all the digits to the right of it.

• If the decimal is absent, start on the Atlantic side at the first nonzero digit and count it and all the digits to the left of it.

Pacific

(Present)

Atlantic

(Absent)

4. 0.080

3. 5,280

2. 402

1. 23.50

Significant FiguresCounting Sig Fig Examples

1. 23.50

2. 402

3. 5,280

4. 0.080

= ____ sig figs

= ____ sig figs

= ____ sig figs

= ____ sig figs

Measurement

Unit Conversions

Significant Figures

5. (15.30 g) ÷ (6.4 mL)

Practice Problems

= _______ g/mL

6. (18.9 g) - (0.86 g)

= _______ g

Scientific Notation• Converting into Sci. Notation:

– Move decimal until there’s 1 digit to its left. Places moved = exponent.

– Large # (>1) positive exponentSmall # (<1) negative exponent

– Only write sig figs.

Write these in Scientific Notation

7. 2,400,000 g

8. 0.00256 kg

9. 7 10-5 km

10. 6.2 104 mm

Practice Problems