How should data be reported in Chemistry?

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# How should data be reported in Chemistry? - PowerPoint PPT Presentation

How should data be reported in Chemistry?. There are two kinds of numbers :. Exact numbers : may be counted or defined (they are absolutely accurate). Numbers obtained from measurements are not exact. These measurements involve estimating. You can report one estimated digit. 6.3 5 or

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### How should data be reported in Chemistry?

There are two kinds of numbers:
• Exact numbers: may be counted or defined (they are absolutely accurate).
Numbers obtained from measurements are not exact.

These measurements involve estimating.

You can report one estimated digit.
• 6.35 or
• 6.36 or
• 6.37
• the last number is the best estimate for the 3 students.
• Two numbers are certain.
• One number is uncertain.
• three significant figures!
How to countsignificant figures ?
• 438 g = 4.38 x 102 3 s.f.

2.2678.42 = 2.67842 x 103 6 s.f.

3. 1.7 2 s.f.

Task 1: Try these! A) Write in scientific notation.B) Determine the number of significant figures
• 506
• 10.05
• 900.43
• 60.00
• 1.09
• 0.06
• 0.00470
How do you round off?
• If the numbers to be discarded are less than 50  leave the last significant number unchanged : 23.31 23
• If the numbers to be discarded are more than 50  add one to the last significant digit : 23.54 24
• If the numbers to be discarded are 50  round off so that the last significant number is an even number : 23.5024

TASK 2: Complete these multiplication and division problems

• 13.7 x 2.5
• 200. x 3.58
• 2.3 x 3.45 x 7.42
• 0.003 / 5
• 5. 89 / 9.0
• 6. 5000 / 55

500 = 1 SF 500. = 3 SF 550 = 2 SF

On a measuring device, for example the measurement of 500 ml in a measuring cylinder, for the purposes of accuracy this is assumed to be an absolute value.

• 0.008 + 0.05
• 4.50 + 3
• 35.89 + 34.6
• 200 – 87.3
• 75.0 – 2.55
• 10.0 – 9.9

Write:

• 35.270 to 3 significant figures
• 0.4140 to 2 significant figures
• 87.257 to 3 significant figures
• 1.350 to 2 significant figures

5. 62.50 to 2 significant figures

Multiple step problems
• When carrying out multiple step problems keep one extra significant figure throughout the whole problem, to reduce rounding errors.
• The final result should be consistent with the number of significant figures given in the experimental measurements.
Converted, Measured and Counted Numbers
• Unit conversions are infinitely accurate. The number of significant figures does not change because conversions are exact values, not measured values.
• Counted numbers are infinitely accurate, such as counting the number of atoms in H2O (there are 3). 3 is an exact value not a measured value. Counting does not need a tool.
• Measuring requires the use of a tool: ruler, scale, balance, graduated cylinder etc… Measured numbers are only as accurate as the tool being used. The number of significant figures should indicate this.
A great website for practicing

Significant figures can be found at:

http://www.sciencegeek.net/Chemistry/taters/sigfigs.htm