Q21Multiple choice
Which of the following terms is unitless?
Show answerHide answer
Correct option: C
Show solutionHide solution
Worked solution
Option (iii) and (iv) are the answers.
NCERT Exemplar · Class 11 · Chemistry
Chapter 1: Some Basic Concepts of Chemistry
Practice each question, then open the answer or worked solution.
Questions
45 questionsQ22Written answer
One of the statements of Dalton’s atomic theory is given below:
“Compounds are formed when atoms of different elements combine in a fixed
ratio”
Which of the following laws is not related to this statement?
(i) Law of conservation of mass
(ii) Law of definite proportions
(iii) Law of multiple proportions
Show solutionHide solution
Worked solution
Option (i) and (iv)
III. Short Answer Type
Q23Written answer
What will be the mass of one atom of C-12 in grams?
Show solutionHide solution
Worked solution
1 mole of carbon atom = 12g= 6.022 × 1023 atoms.
Q24Written answer
How many significant figures should be present in the answer to the following
calculations?
2.5 1.25 3.5/2.01
Show solutionHide solution
Worked solution
Two significant figures should be present in this.
Since the least number of significant figures from the given figure is 2 (in 2.5 and 3.5).
Q25Written answer
What is the symbol for the SI unit of a mole? How is the mole defined?
Show solutionHide solution
Worked solution
The symbol for the SI unit of the mole is mol. A mole is defined as the amount of substance that contains as many entities as there are atoms in 12g carbon.
Q26Written answer
What is the difference between molality and molarity?
Show solutionHide solution
Worked solution
Molarity is the number of moles of solute dissolved in 1 litre of the solution. Molality is the number of moles of solute present in 1kg of the solvent.
Q27Written answer
Calculate the mass percent of calcium, phosphorus and oxygen in calcium
phosphate Ca3(PO4)
Show solutionHide solution
Worked solution
Molecular mass of Ca3(PO4) = 3*40+2*31+8*16 =310
Mass per cent of Ca = 3*40/310*100 = 38.71%
Mass per cent of P = 2*31/310*100 = 20%
Mass per cent of O = 8*16/310 = 41.29%
Q28Written answer
45.4 L of dinitrogen reacted with 22.7 L of dioxygen and 45.4 L of nitrous
oxide was formed. The reaction is given below:
2N2(g) + O2(g) → 2N2O(g)
Which law is being obeyed in this experiment? Write the statement of the law?
Show solutionHide solution
Worked solution
The above experiment proves Gay-Lussac’s law which states that gases combine or produced in a chemical reaction in a simple whole-number ratio by volume provided that all gases are the same temperature and pressure.
Q29Written answer
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 whole-number ratio.
(a) Is this statement true?
(b) If yes, according to which law?
(c) Give one example related to this law.
Show solutionHide solution
Worked solution
(a) Yes, the statement is true.
(b) According to the law of multiple proportions
(c), Hydrogen and oxygen react to form water and hydrogen peroxide
H2 + 1/2O2 → H2O
H2 + O2 → H2O2
Masses of oxygen which combine the fixed mass of hydrogen are in the ratio 16:32 or 1
Q30Written answer
Calculate the average atomic mass of hydrogen using the following data :
| Isotope % | Natural abundance | Molar mass |
| 1H | 99.985 | 1 |
| 2H | 0.015 | 2 |
Show solutionHide solution
Worked solution
Average atomic mass = 99.985*1+0.015*2/100
=099.985*1+0.015*2/100
=1.00015u