SI units and sig figs

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# SI units and sig figs - PowerPoint PPT Presentation

SI units and sig figs. SI (système internationale). Precision: the reproducibility, under the same conditions, of a measurement . These points are precise with one another but not accurate. Accuracy: the closeness of a measurement to the true value of what is being measured.

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### SI units and sig figs

Precision: the reproducibility, under the same conditions, of a measurement .
• These points are precise with one another but not accurate.
Accuracy: the closeness of a measurement to the true value of what is being measured.
• Ex: the bullseye would be the true value, so these points are accurate.
Using sig figs: The Rules!
• Digits from 1-9 are always significant.
• Zeros between two other significant digits are always significant
• One or more additional zeros to the right of both the decimal place and another significant digit are significant.
• Zeros used solely for spacing the decimal point (placeholders) are not significant.
Multiplying and Dividing
• RULE: your answer may only show as many sig figs as the multiplied or divided measurement showing the least number of significant digits.
• Example: 22.37 cm x 3.10 cm = 69.3 only 3 sig figs allowed.
• RULE: your answer can only show as many decimal places as the measurement having the fewest number of decimal places.
• Example:

3.76 g + 14.83 g + 2.1 g = 20.7 g

Scientific Notation
• Scientists have developed a shorter method to express very large numbers.
• Scientific Notation is based on powers of the base number 10.
123,000,000,000 in s.n. is 1.23 x 1011
• The first number 1.23 is called the coefficient. It must be between 1 - 9.99
• The second number is called the base . The base number 10 is always written in exponent form. In the number 1.23 x 1011 the number 11 is referred to as the exponent or power of ten.
To write a large number in scientific notation:ex: 36 000
• First put the decimal after the first digit and drop the zeroes. Ex: 3.6
• Next, count the number of places from the decimal to the end of the number. Ex: 4
• Finally, put it together. Ex: 3.6 x 104
To write a small number in s.n.ex: 0.00064
• First move the decimal after the first real number and drop the zeroes. Ex: 6.4
• Next, count the number of places moved from the original decimal spot to the new decimal spot. Ex: 4
• Numbers less than 1 will have a negative exponent. Ex: -4
• Finally, put it together. Ex: 6.4 x 10-4
Significant Digits
• When using a measuring device, there is a last number estimation.
• The accuracy of the sig figs depends upon the measuring device.
• Ex: a ruler.