Page 153 - Chemistry ICSE Class X

Mole Concept and Stoichiometry 139
16. In certain mass of a gas, the number of atoms and the number of molecules are equal. What conclusion do you
draw from this observation?
17. &GſPG VJG VGTO OQNGEWNCT OCUU *QY FQGU OQNGEWNCT OCUU FKHHGT HTQO OQNCT OCUU!
18. Write the relationship between the number of moles (n), mass (m) and the molar mass (M) of a substance.
19. &GſPG C OQNGEWNCT HQTOWNC D GORKTKECN HQTOWNC
20. Write the empirical formula of (a) diamond (b) graphite.
21. Why is it not correct to write molecular formula of an ionic compound?
22. What is meant by the composition of a compound?
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23. How much a sample consisting of 1.5 × 10 molecules of carbon dioxide at STP would weigh? [Ans. 1100 g]
24. Calculate the number of molecules in 8.8 g of carbon dioxide at STP. [At. masses: C = 12, O = 16] [Ans. 1.2 × 10 ]
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25. If you are asked to bring 10 moles of water, how much water would you bring? Also calculate the number of
water molecules (H O) in this sample of water. [Ans. 180 g, 6.02 × 10 ]
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–6
26. You are asked by your teacher to distribute 10 moles of sugar (C H O ) to each of your 100 classmates. How
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many molecules of sugar each one of you would get? [Ans. 6.02 × 10 ]
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27. The mass of 5.6 L of a gas under STP is 12 g. What is the relative molecular mass and molar mass of the gas?
[Ans. 48, 48 g/mol]
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28. Calculate the number of moles in 3.01 × 10 atoms of calcium. [Ans. 0.5 mol]
29. Which of the following weigh the most?
(a) 2 g-atoms of nitrogen (b) 25 g of iron
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(c) 2 × 10 atoms of carbon (d) 22.4 L of CO at NTP [Ans. 22.4 L of CO has the maximum mass]
2
2
30. 20 mL of hydrogen, 10 mL of carbon monoxide and 20 mL of oxygen are exploded in an eudiometer. What will
be the volume and composition of the mixture after cooling it to room temperature?
[Ans. V total = 15 mL, V = 5 mL, V CO 2 =10 mL]
O 2
31. Acetylene burns in air (containing 20% oxygen) in accordance with the reaction,
2C H (g) + 5O (g) o 4CO (g) + 2H O
2
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2
2
2
from air
Find the volume of the air required to completely burn 50 mL of acetylene. [Ans. 625 mL]
32. Under the similar conditions of temperature and pressure, a gas cylinder can hold 20 g of hydrogen, 180 g of gas
A and 240 g of gas B. Calculate the relative molecular masses of A and B. [Ans. A = 18, B = 24]
33. A gas having the formula (CO) has vapour density equal to 70. Find the value of x. [Ans. x = 5]
x
34. Calculate the mass of 5.5 billion molecules of water. Atomic masses are : H = 1.0 u, O = 16.0 u. [Ans. 1.6 × 10 –13 g].
35. How much mass (in grams) is represented by the following?
(a) 0.1 mol of NaCl (b) 0.5 mol of C
(c) 5.6 L of CO at NTP [Ans. (a) 5.85 g (b) 6 g (c) 11 g]
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36. The molecular formula of a compound is CH OH. Calculate its molecular mass in atomic mass units. Also write
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down its molar mass. Atomic masses are : H = 1 u, C = 12 u, O = 16 u. [Ans. 32 u; 32 g/mol]
37. Calculate the simplest formulae (empirical formula) of the substances having the following compositions (by
mass percentage).
(a) C = 75%, H = 25% (b) H = 6.25%, N = 43.75%, O = 50.0%
38. Determine the empirical formula of the substance having the following composition: C = 49.3%, H = 9.6%,
N = 19.2% and O = 21.9%.
Relative atomic masses are: C = 12, H = 1, N = 14 and O = 16. [Ans. C 3 H 7 NO]
39. A compound contains 82.98% potassium and 17.02% oxygen. What is its empirical formula? Relative atomic
masses are, K = 39, O = 16. [Ans. K O]
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40. 10 g of phosphorus on reacting with oxygen produced 17.77 g of phosphorus oxide. What is its empirical formula?
Relative atomic masses are : P = 31, O = 16. [Ans. P 2 O 3 ]
41. A compound on analysis gave the following results: C = 54.54%, H = 9.09% and O = 36.26%
The relative molecular mass of the compound is 88. Determine the molecular formula of the compound.
[Ans. C 4 H 8 O 2 ]