Some Basic Concepts of Chemistry — Chemistry STD 11 Science — Question

The uncertainty in the experimental or the calculated values is indicated by mentioning the number of significant figures. Significant figures are meaningful digits which are known with certainty plus one which is estimated or uncertain. The uncertainty is indicated by writing the certain digits and the last uncertain digit. there are certain rules for determining the Number of significant figures. These are Stated below:

• All non-zero digits are significant. For Example in 285cm, there are three Significant figures and in 0.25 mL, there are two significant figures.
• Zeros preceding to first non-zero digit are not significant. such zero indicates the position of decimal point. thus, 0.03 has one significant figure and 0.0052 has two significant figures.
• Zeros between two non-zero digits are significant. thus, 2.005 has four Significant figures.
• Zeros at the end or right of a number are significant, provided they are on the right side of the decimal point. For example, 0.200 g has three significant figures. But, if otherwise, the terminal zeros are not significant if there is no decimal point.

Precision refers to the closeness of various measurements for the same quantity. However, accuracy is the agreement of a particular valueto the true value of the result.

LAWS OF CHEMICALCOMBINATIONS- The combination of elements to form compounds is governed by the following five basic laws.

1. Law of Conservation of Mass-This law was put forth by Antoine Lavoisierin 1789. He performed careful experimental studies for combustion reactions and reached to the conclusion that in all physical andchemical changes, there is no net change inmassduring the process. Hence, he reachedto the conclusion that matter can neither becreated nor destroyed. This is called ‘Law ofConservation of Mass’.
2. Law of Definite Proportions-This law was given by, a French chemist, Joseph Proust. He stated that a given compound always contains exactly the same proportion of elements by weight.
3. Law of Multiple Proportions-This law was proposed by John Dalton. According to this law, if two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in the ratio of small whole numbers. For example, hydrogen combines with oxygen to form two compounds, namely, water and hydrogen peroxide.

Hydrogen + Oxygen→ Water

2g                 16g       18g

Hydrogen + Oxygen → Hydrogen Peroxide

    2g                  32g        34g

Here, the masses of oxygen (i.e., 16 g and 32 g), which combine with a fixed mass of hydrogen (2g) bear a simple ratio, i.e., 16:32 or 1:2.

4. Gay Lussac’s Law of Gaseous Volumes-This law was given by Gay Lussac in 1808. Heobserved that when gases combine or are produced in a chemicalreaction they do so in asimple ratio by volume,provided all gases are at the same temperature and pressure.
5. Avogadro’s Law – In 1811, Avogadro proposed that equal volumes of all gases at the same temperature and pressure should contain equal number of molecules.

In 1808, Dalton published ‘A New System of Chemical Philosophy’, in which he proposed the following :

1. Matter consists of indivisible atoms.
2. All atoms of a given element have identical properties, including identical mass. Atoms of different elements differ in mass.
3. Compounds are formed when atoms of different elements combine in a fixed ratio.
4. Chemical reactions involve reorganisati on of atoms. These are neither created nor destroyed in a chemical reaction.

1. … refers to the closeness of variousmeasurements for the same quantity.

1. Accuracy
2. Reliability
3. Precision
4. Uncertainty

2. Law of Conservation of mass was put forth by ….in 1789.

1. Joseph Proust
2. Antoine Lavoisier
3. Joseph Louis
4. Gay Lussac

3. Which of the following number has twosignificant figures.

1. 0.0052
2. 052
3. 52
4. 0052

4. … is the agreement of a particular valueto the true value of the result.

1. Accuracy
2. Reliability
3. Precision
4. Uncertainty

5. Law of Multiple Proportions proposed by....

1. Joseph Proust
2. Antoine Lavoisier
3. Joseph Louis
4. John Dalton