Standards For Measurement:
Units of Measurement: (Quantitative Observation)

Two Parts:
1. Number
2. Units

A measurement is expressed by a numerical value together with a unit of that measurement.
Ex. 70.0 kilograms = 154 lbs.

Number obtained from a measurement are never exact values. They always have some degree of uncertainty due to limitations of the measuring instrument and the skill of the individual making the instrument. The numerical value recorded for a measurement should give some indication of its reliability (precision).

Precision:
-Refers to the degree of agreement among several measurements of the same quantity.

Accuracy:
-Refers to the agreement of a particular value with the true value.

Significant Figures:
-Are those digits in the measured number that include all certain digit plus a final one that is somewhat uncertain.

Guidelines for Using Significant Figures:
1. Any digit that is not zero is significant.
845 = 3 s.f. 1.234 = 4 s.f.

2. Zeros between nonzero digits are significant.
606 = 3 s.f. 40,501 = 5 s.f.

3. Zeros to the left of the first non-zero digit are not significant.
0.08 = 1 s.f. 0.00000349 = 3 s.f.

4. If a number is greater than 1, then all zeros to the right of the decimal point is counted as significant figures.
2.0 = 2 s.f. 3.040 = 4 s.f.
*- If a number is less than 1, then only zeros that are the end of the number and the zeros that are between nonzero digits are significant.
0.090 = 2 s/f 0.3005 = 4 s.f.

5. For numbers that do not contain trailing zeros may or may not be significant.
400 --------------- 4 x 10^2 = 1 s.f.
4.0 x 10 ^2 = 2 s.f.
4.00 x 10^2 = 3 s.f.

Significant Figures in Calculations:
1. When multiplying or dividing the quantities, give as many significant figures in the answer as there are in the quantity with the fewest number of significant figures.

2. When adding or subtracting quantities, give the same number of the decimal places in the answer as there are in the quantity with the fewest number of decimal places.

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Chemistry is the science of the composition, structure, properties, and reactions of matter, especially of atomic and molecular systems.