Some Basic Concepts of Chemistry Chemistry - Some Basic Concepts of Chemistry

Avogadro's law finds an application in the determination of :

In chulhas, gaps are left between the logs :

Which one of the following will have the largest number of atoms?
1g Li(s)

What is the SI unit of mass? How is it defined?

Convert the following into basic units:
15.15pm

Convert the following into basic units:
28.7pm

How many significant figures are present in the following?
5005

Calculate the number of atoms of the following:
52 moles of Ar

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

Read the passage given below and answer the following questions from 1 to 5.
Chemistry is the science of molecules and theirtransformations. It is the science not so much of the one hundred elements but of the infinite variety of molecules thatmay be built from them. Chemistry plays a central role in science andis often intertwined with other branches ofscience.to understand thebasic concepts of chemistry, which begin withthe concept of matter. Let us start with thenature of matter. matter can exist in threephysical states viz. solid, liquid and gas.Particles are held very close to each otherin solids in an orderly fashion and there is notmuch freedom of movement. In liquids, theparticles are close to each other but they canmove around. However, in gases, the particlesare far apart as compared to those present insolid or liquid states and their movement iseasy and fast. different states of matter exhibitthe following characteristics:

1. Solids have definite volume and definiteshape.
2. Liquids have definite volume but do nothave definite shape. They take the shapeof the container in which they are placed.
3. Gases have neither definite volume nordefinite shape. They completely occupy thespace in the container in which they are placed.

Matter can be classified as mixture or pure substance. A mixture may be homogeneous or heterogeneous. Pure substances can further be classified into elements and compounds. Particles of an element consist of only one type of atoms. These particles may exist as atoms or molecules. When two or more atoms of different elements combine together in a definite ratio, the molecule of a compound is obtained.
Every substance has unique or characteristic properties. These properties can be classified into two categories — physical properties, such as colour, odour, melting point, boiling point, density, etc., and chemical properties, like composition, combustibility, ractivity with acids and bases, etc. Physical properties can be measured or observed without changing the identity or the composition of the substance. The measurement or observation of chemical properties requires a chemical change to occur. Measurement of physical properties does not require occurance of a chemical change.

1. Which of the following state of matter have definite volume but do not have definite shape?

1. Solid
2. Liquid
3. Gas
4. Plasma

2. Particles are held very close to each other in … in an orderly fashion and there is not much freedom of movement.

1. Liquid
2. Gas
3. Solid
4. Plasma

3. Particles of …. consist of only one type of atom.

1. Compound
2. Mixture
3. Element
4. All the above

4. Water molecule comprises …hydrogen atoms and … oxygen atom.

1. One, two
2. Three, one
3. One, three
4. Two, one

5. Which of the following is not an example of Physical Properties of substance.?

1. Odour
2. Melting point
3. Density
4. Composition

A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0L (measured at STP) of this welding gas is found to weigh 11.6g. Calculate.

1. Empirical formula.
2. Molar mass of the gas.
3. Molecular formula.

Express the following in the scientific notation:

1. 0.0048
2. 234, 000
3. 8008
4. 500.0
5. 6.0012