Question:
Can atoms of an element exist independently? Give examples of elements which exist in atomic form. Give examples of elements that do not exist in atomic form.
Answer:
Atoms of most elements do not to exist independently. There are few elements like Helium (He), Argon (Ar), Krypton(Kr), Radon(Rn), Neon(Ne) and Xenon(Xe) - collectively called noble gases, exist at atomic form. Atoms of other elements combine together and form molecules and
ions. These molecules or ions aggregate in large numbers to form the matter. e.g.
- Hydrogen (H2), Oxygen(O2), Nitrogen(N2) etc. atoms of the same element combine together and form molecules.
- Water (H2O), Carbon Dioxide(CO2), Common Salt(NaCl) etc. atoms of different elements combine together and form molecules.
- Na+, Cl-, Cu2+ etc. atoms of elements form ions in aqueous solutions.
Atoms and Molecules
Q 1.
What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Q 2.
An element Z has a valency of 3. What is the formula of oxide of Z ?
Q 3.
What is the numerical value of Avogadro number ?
Q 4.
Write the full form of IUPAC.
Q 5.
If one mole of nitrogen molecules weighs 28 g, calculate mass of one molecule of nitrogen in grams.
Q 6.
How many moles are there in 4.6 gms of Sodium(Na)?
Q 7.
How many moles of calcium carbonate (CaCO
Q 8.
What is meant by atomicity ? Explain with two
Q 9.
What do we call those particles which are formed :
(a)by the gain of electrons by atoms ?
(b)by the loss of electrons by atoms ?
Q 10.
What is the difference between a cation and an anion ? Explain with examples. Using this information, write down the formulae of:
(i) Sodium sulphide
(ii) Copper nitrate
Q 11.
How many moles are 3.6 g of water ?
Q 12.
Calculate the number of molecules in 4 g of oxygen.
Q 13.
Show by means of calculations that 5 moles of CO, and 5 moles of H
do not have the same mass. How much is the difference in their masses ?
Q 14.
Is there any exception to law of conservation of mass?
Q 15.
What are the building blocks of matter ?
Q 16.
What is an ion ? How is an ion formed ? Explain with the help of two examples of different ions.
Q 17.
What name is given to the number 6.022 x 10
Q 18.
What is the mass of 0.2 mole of oxygen atoms ?
Q 19.
State Law of constant proportions. Explain with an example.
Q 20.
Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?
Q 21.
Fill in the blanks:
The _______________ of a compound is a symbolic representation of its composition.
Q 22.
Fill in the blanks:
According to law of definite proportions, in a chemical substance the elements are always present in __________ proportions by mass.
Q 23.
How many atoms are present in a
(i) H
2S molecule and
(ii) PO
43- ion?
Q 24.
The molecular formula of glucose is C
Q 25.
How many moles are there in 34.5 g of sodium ? (Atomic mass of Na = 23 u)
Q 26.
If 16 g of oxygen contains 1 mole of oxygen atoms, calculate the mass of one atom of oxygen.
Q 27.
Explain with example that law of conservation of mass is valid for chemical reactions.
Q 28.
Fill in the blanks:
Atoms can be observed using ____________ Microscope.
Q 29.
Magnesium is two times heavier than C-12 atom, what shall be the mass of Mg atom in terms of atomic mass units? (Given mass of C-12 atom = 12u)
Q 30.
What is relative atomic mass of an element? How it is related to atomic mass unit?
Q 31.
Give four examples of diatomic molecules.
Q 32.
What is the molecular mass of a substance?
Q 33.
Calculate the molecular masses of H
2, O
2, Cl
2, CO
2, CH
4, C
2H
6, C
2H
4, NH
3, CH
3OH.
Q 34.
Dalton's atomic theory says that atoms are indivisible. Is this statement still valid ? Give reasons for answer.
Q 35.
Calculate the molecular mass of ethanoic acid, CH
Q 36.
Calculate the molecular mass of chloroform (CHC1
Q 37.
Name the elements water is made of. What are the valencies of these elements ? Work out the chemical formula for water.
Q 38.
An element X has a valency of 4 whereas another element Y has a valency of 1. What will be the formula of the compound formed between X and Y ?
Q 39.
Work out the formulae for the following compounds :
(a) Sodium oxide
Q 40.
What are (ionic compounds, and (ii) molecular compounds ? Give two examples of each type of compounds.
Q 41.
How many atoms are present in one gram atomic mass of a substance ?
Q 42.
Calculate the mass in grams of 0.17 mole of hydrogen sulphide, H
Q 43.
(a) What is meant by 'a mole of carbon atoms' ?
(b)(b) Which has more atoms, 50 g of aluminium or 50 g of iron ? Illustrate your answer with the help of calculations.
(Atomic masses : A1 = 27 u ; Fe = 56 u)
Q 44.
(a) Define gram atomic mass of a substance.How much is the gram atomic mass of oxygen ?
(b) How many moles of oxygen atoms are present in one mole of the following compounds ?
(i)Al
(ii) co
2(iii) C1
(iv) H
(p)A1
Q 45.
Which postulate of Dalton’s atomic theory can explain the law of definite proportions?
Q 46.
Fill in the blanks:
_________ are building block of all matter.
Q 47.
What is the atomic mass unit?
Q 48.
How do we know the presence of atoms if they do not exist independently for most of the elements?
Q 49.
Calculate the molecular masses of the following :
(a) Hydrogen, H
Q 50.
Name the following compounds. Also write the symbols/formulae of the ions present in them :
(a) CuSO
