chemistry

SOME BASIC CONCEPTS OF CHEMISTRY

Two Marks questions with answers

1.  Give the two points of differences between homogeneous and heterogeneous mixtures.

Ans.

Homogeneous mixture				Heterogeneous mixture
1.	Homogeneous		mixtures	1.	Heterogeneous  mixtures  consist  of
	have the same composition				two  or  more  parts  (phases),  which
	throughout the sample.				have different compositions.
				2.	These	mixtures	have	visible
2.	The  components  of  such				boundaries of separation between the
	mixtures	cannot	be  seen		different constituents and can be seen
	under	a	powerful		with the naked eye
	microscope.

2.     Copper oxide obtained by heating copper carbonate or copper nitrate contains copper and oxygen in the same ration by mass. Which law is illustrated by this

observation? State the law.

Ans.Law of Definite Proportions This law states that: A chemical compound always consists of the same elements combined together in the same ratio, irrespective of the method of preparation or the source from where it is taken.

3.	Write the empirical formula of the following:
	(a) N2O4	(b) C6H12O6  (c) H2O	(d) H2O2
Ans.  (a)NO2		(b) CH2O    (c) H2O		(d) HO
4.	Briefly explain the difference between precision and accuracy.

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

5. Define the law of multiple proportions. Explain it with one example. Ans.When two elements combine to form two or more compounds, then the different masses of one element, which combine with a fixed mass of the other, bear a simple ratio to one another. For example- carbon combines

with oxygen to form two compounds CO and CO2.
Compound	CO	CO2
Mass of C	12	12
Mass of O	16	32

Masses of oxygen which combine with a fixed mass of carbon (12g) bear a simple ratio of 16:32 or 1:2.

6.   Chlorine has two isotopes of atomic mass units 34.97 and 36.97. The relative abundance of the isotopes is 0.755 and 0.245 respectively. Find the average atomic mass of chlorine.

Ans. Average atomic mass = 34.97 x 0.755 +36.97 x 0.245 = 35.46 u

7.  Calculate the percentage composition		of water.
Ans. Mass % of an element = mass of that element in the compound × 100
molar mass of the compound
Molar mass of water = 18.02 g
Mass % of hydrogen = 2× 1.008 × 100
18.02
= 11.18
Mass % of oxygen = 16.00 × 100		= 88.79
18.02

8.  State the number of significant figures in each of the following:

(i)   208.91 (ii) 0.00456 (iii) 453 (iv)  0.346

Ans.

(i)   208.91 has five significant figures.

(ii)   0.00456 has three significant figures.

(iii)   453 has three significant figures.

(iv)  0.346 has three significant figures.

8.     Express the results of the following calculations to the appropriate number of significant figures.

=  0.2615 x 10-4 = 0.3 x 10^-4

9.     How are 0.50 mol Na2CO3 and 0.50 M Na2CO3 different? Ans. Molar mass of Na2CO3= 2 x 23 +12 + 3 x 16 = 106 g / mol

0.50 molNa2CO3means 0.50 x 106 = 53 g

0.50 M Na2CO3 means 0.50 mol i.e. 53 g of Na2CO3 are present in I L of the solution.