Some Basic Concepts of Chemistry

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Many times in the study of chemistry, one has to deal with experimental data as well as theoretical calculations. The measurement uncertainty is often taken as the standard deviation of a state-of-knowledge probability distribution over the possible values.

Dimensional Analysis During calculations generally there is a need to convert units from one system to other. This is called factor label method or unit factor method or dimensional analysis.

For example- 5 feet and 2 inches (height of an Indian female) is to be converted in SI unit:

In which any number can be represented in form N × 10n (Where n is an exponent having positive or negative values and N can vary between 1 to 10). E.g. we can write 232.508 as 2.32508 x10² in scientific notation. Similarly, 0.00016 can be written as 1.6 x 10^–4.

Precision refers to the closeness of various measurements for the same quantity.

Accuracy is the agreement of a particular value to the true value of the result.

The reliability of a measurement is indicated by the number of digits used to represent it. To express it more accurately we express it with digits that are known with certainty. These are called as Significant figures. They contain all the certain digits plus one doubtful digit in a number.

Rules for Determining the Number of Significant Figures

1. All non-zero digits are significant. For example, 6.9 have two significant figures, while 2.16 have three significant figures. The decimal place does not determine the number of significant figures.
2. A zero becomes significant in case it comes in between non zero numbers. For example, 2.003 has four significant figures, 4.02 has three significant figures.
3. Zeros at the beginning of a number are not significant. For example, 0.002 has one significant figure while 0.0045has two significant figures.
4. All zeros placed to the right of a number are significant. For example, 16.0 has three significant figures, while 16.00has four significant figures. Zeros at the end of a number without a decimal point are ambiguous.
5. In exponential notations, the numerical portion represents the number of significant figures. For example, 0.00045 is expressed as 4.5 x 10^-4 in terms of scientific notations. The number of significant figures in this number is 2, while Avogadro's number (6.023 x 10²³) it is four.
6. The decimal point does not count towards the number of significant figures. For example, the number 345601 has six significant figures but can be written in different ways, as 345.601 or 0.345601 or 3.45601 all having the same number of significant figures.

Retention of Significant Figures - Rounding off Figures

1. The rounding off procedure is applied to retain the required number of significant figures.
2. If the digit coming after the desired number of significant figures happens to be more than 5, the preceding significant figure is increased by one, 4.317 is rounded off to 4.32.
3. If the digit involved is less than 5, it is neglected and the preceding significant figure remains unchanged, 4.312 is rounded off to 4.31.
4. If the digit happens to be 5, the last mentioned or preceding significant figure is increased by one only in case it happens to be odd. In case of even figure, the preceding digit remains unchanged. 8.375 is rounded off to 8.38 while8.365 is rounded off to 8.36.