Chapter 1:Unit 9. Exact number and Measured number

Exact number and Measured number

An exact number results from counting objects or is a part of a definition. For example, 30 students present in class. 30 is the exact number. There are 60 minutes in 1 hour. 60 is the exact number. A measured number results from a measurement or observation and contains some uncertainty also called Inexact number. Limitation of the measuring device and limited powers of the observation of the individual making the measurement involving a degree of uncertainty or error.  That is why measured number are sometimes called approximate number.

Example of exact number: 30 students in class, 12 oranges. Example of measured number: the child weights 30.5 lbs, The height of Mount Everest is 8,848 meters.

Any exact doesn’t have any uncertainty in it. In case of measured number always, the last digit is uncertain. The uncertain number depends on the calibration of the device.

In a measured number always, all digits are certain except the last one. The last digit in a measured number of based on instrumental precision and human eye estimation. It is called uncertain digit or estimated digit.  Certain and uncertain digits together make significant figures.

Watch the following video about certain and uncertain digits.

Following tutorial may help you to understand the concept.

Questions:

1. Indicate whether the following numbers are exact or inexact?
   a)The length of a river
   b) Conference attendees
2. Go to the following link Take the quiz: https://teachchemistry.org/periodical/issues/september-2017/measuring-volume

3, What measurement can be shown with following ruler?

Ans:

1. a) measured

          b)exact

2. Quiz
3. 14.30 cm

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