# Some basic concepts of Chemistry - III

1. Mass of a substance is the amount of matter present in it while weight is the force exerted by gravity on an object. The mass of a substance is constant whereas its weight may vary from one place to another due to change in gravity

2. Volume has the units of (length)3. So in SI system, volume has units of m3. A common unit, litre (L) which is not an SI unit, is used for measurement of volume of liquids. 1 L = 1000 mL , 1000 cm3 = 1 dm33. Density of a substance is its amount of mass per unit volume SI units of density kg m-3 This unit is quite large and a chemist often expresses density in g cm-34. There are three common scales to measuretemperature — °C (degree celsius), °F (degree Fahrenheit) and K (Kelvin). Here, K is the SI unit.5. The Kelvin scale is related to Celsius scale as follows :K =°C+273.15

3. The °F scale is related to Celsius scale as follows

4. In scientific notation (exponential Notation) any number can be represented in the form N × 10n where n is an exponent having positive or negative values and N can vary between 1 to 10. Thus, we can write 232.508 as 2.32508 X102 in scientific notation. Note that while writing it, the decimal had to be moved to the left by two places and same is the exponent (2) of 10 in the scientific notation. Similarly, 0.00016 can be written as 1.6 X10-4. Here the decimal has to be moved four places to the right and (4) is the exponent in the scientific notation

5. Significant figures are meaningful digits which are known with certainty. The uncertainty is indicated by writing the certain digits and the last uncertain digit. Thus, if we write a result as 11.2 mL, we say the 11 is certain and 2 is uncertain and the uncertainty would be ± 1 in the last digit. Unless otherwise stated, an uncertainty of +1 in the last digit is always understood.

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