What is an exact number?

A counted or defined value with no measurement uncertainty, such as a stable count or the defined relationship $1\text{ in}=2.54\text{ cm}$.

Why do measured values have uncertainty?

Instrument resolution, user technique, and measurement conditions limit how precisely a value can be determined.

What are significant figures in a measured value?

All reported digits, including the final estimated digit.

How do accuracy and precision differ?

Accuracy is closeness to an accepted value; precision is agreement among repeated measurements.

How is density calculated?

Divide an object's mass by its volume.

General chemistryLesson 5 of 6 in the course Essential Ideas

Measurement Uncertainty, Significant Figures, and Accuracy and Precision

5 flashcards · 5 quiz questions · 5 match pairs · 5 fill-in sentences · English

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Summary

Measurements are essential in chemistry, but measured values are never perfectly certain. This lesson distinguishes exact numbers from measured numbers and explains why every measurement has uncertainty.

An exact number has no uncertainty. Numbers obtained by counting objects are exact, as long as the number of objects does not change during counting. For example, counting the eggs in a carton gives an exact count.

All flashcards

What is an exact number?: A counted or defined value with no measurement uncertainty, such as a stable count or the defined relationship $1\text{ in}=2.54\text{ cm}$ .
Why do measured values have uncertainty?: Instrument resolution, user technique, and measurement conditions limit how precisely a value can be determined.
What are significant figures in a measured value?: All reported digits, including the final estimated digit.
How do accuracy and precision differ?: Accuracy is closeness to an accepted value; precision is agreement among repeated measurements.
How is density calculated?: Divide an object's mass by its volume.

Quiz questions

Which quantity is exact if the objects remain unchanged during counting?Answer: The number of eggs counted in a carton
How many significant figures are in $0.008324$ ?Answer: Four
What should be reported for $1.0023\text{ g}+4.383\text{ g}$ using the addition and subtraction rounding rule?Answer: $5.385\text{ g}$
Rounding $92.85$ to three significant figures using the exact-tie rule gives which result?Answer: $92.8$
A graduated cylinder's liquid level rises from $13.5\text{ mL}$ to $22.4\text{ mL}$ when an object is submerged. What is the object's volume?Answer: $8.9\text{ mL}$

Based on Chemistry 2e by Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, PhD (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.

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